UC-NRLF 


Elementary 
Organic  Analysis 


P.G,  BENEDICT 


The  Chemical  Publishing  Co. 


Elementary  Organic  Analysis. 


Elementary  Organic 
Analysis 


The   Determination  of   Carbon  and 
Hydrogen 


BY 


FRANCIS  GANG  BENEDICT,  PH.D., 

Instructor  in  Chemistry  in  Wesleyan  University 


EASTON,  PA.: 

THE  CHEMICAL  PUBLISHING  CO. 
1900. 


COPYRIGHT,  1900,  BY  EDWARD  HART. 


B 


PREFACE 

Perhaps  no  analytical  operation  is  at  once  so  funda- 
mentally important  and  exasperatingly  vexatious  as 
the  organic  combustion.  Notwithstanding  this  fact} 
save  for  the  meager  statements  in  one  or  two  of  the 
larger  books  on  organic  chemistry,  no  description  of 
the  process  of  the  determination  of  carbon  and  hydro- 
gen is  accessible  to  most  students.  As  a  rule  a  knowl- 
edge of  *  the  operation  is  chiefly  obtained  by  word  of 
mouth. 

This  little  manual  is  presented  in  the  hope  that  the 
descriptions  of  processes  here  recorded  will  aid  in 
making  this  method  of  analysis  more  familiar  and 
more  satisfactory. 

Usually  very  little,  if  any,  discrimination  is  exer. 
cised  in  burning  the  compounds  obtained  in  organic 
research,  and  experiment  alone  is  relied  upon  to  secure 
the  proper  conditions  for  complete  combustion.  It  is 
hoped  that  the  different  cases  cited  in  the  latter  part 
of  the  manual  will  aid  in  giving  some  clue  to  the  treat- 
ment necessary  for  many  compounds,  thereby  saving 
time  and,  more  frequently,  valuable  material. 

While  an  attempt  has  been  made  to  describe  all  op- 
erations commonly  used  it  is  obviously  impossible  not 
to  give  fuller  consideration  to  such  modifications  of 
the  general  method  as  have  been  suggested  by  an  ex- 
perience with  over  two  thousand  combustions.  Ac- 


vi  PREFACE 

cordingly  these  modifications  are  treated  in  detail  and 
as  a  general  rule  recommended  in  preference  to  the 
older  manipulations. 

For  the  painstaking  care  and  numerous  suggestions 
of  Mr.  Emil  Osterberg,  assistant  in  this  laboratory, 
whose  experimental  skill  has  contributed  greatly  to 
many  of  the  modifications  here  presented,  the  writer 
is  extremely  grateful. 
MIDDI.ETOWN,  CONN. 


TABLE  OF  CONTENTS 

Introduction  i 

Preparation  of  oxygen  2 

Compressed  oxygen  -       3 

Gasometers  or  gas-holders    -  6 

Air   -  ii 

Purifying  apparatus  12 

Rubber  tubing  and  stoppers  1 7 

Combustion  furnaces     -  18 

Combustion  tubes  21 

Oxidizing  agents  24 

Filling  the  combustion  tube     -  -     27 

Boats     -  29 

Absorbing  agents  -     31 

Absorbing  apparatus  33 

Cleaning  and  weighing  absorbing  apparatus    -  42 

Weight  of  material  used  45 

Burning  out  the  combustion  tube  -     45 

General  process  of  the  combustion  49 

Combustion  of  nitrogenous  substances  58 

Combustion  of  bodies  containing  the  halogens     f  65 

Combustion  of  bodies  containing  sulphur  -     68 

Combustion  of  bodies  containing  the  alkali  metals  70 

Combustion  of  difficultly  combustible  bodies  -  70 

Combustion  of  liquids  and  volatile  bodies    -  73 

Combustion  of  explosive  bodies  80 

Calculation  of  results    -  80 

Appendix  82 

Index     -  83 


INTRODUCTION 

The  analysis  of  organic  compounds  requires  the  de- 
termination of  but  few  elements,  the  most  important 
of  which  are  carbon,  hydrogen,  and  nitrogen.  This 
book  has  to  do  only  with  the  determination  of  carbon 
and  hydrogen  in  organic  compounds.  The  method 
consists  of  converting  all  the  carbon  to  carbon  dioxide, 
all  the  hydrogen  to  water,  and  absorbing  and  weigh- 
ing these  products. 

In  bodies  containing  nitrogen,  care  is  taken  to  pro- 
vide that  none  of  its  compounds  are  retained  in  the 
systems  used  to  absorb  water  or  carbon  dioxide.  In 
general  the  nitrogen  escapes  in  the  gaseous  form. 

The  oxidation  of  the  carbon  and  hydrogen  may  be 
effected  by  mixing  the  material  intimately  with  me- 
tallic oxides,  such  as  cupric  or  mercuric  oxide,  or  with 
oxygenated  salts,  such  as  potassium  chlorate,  potassium 
dichromate,  or  lead  chromate.  In  some  methods  use 
is  made  of  a  current  of  air  or  oxygen  to  facilitate  the 
burning  of  refractory  substances  and  to  regenerate  the 
oxidizing  agents.  The  property  of  platinum  and  pal- 
ladium of  condensing  large  quantities  of  oxygen  on 
the  surface  of  the  finely-divided  metal  has  also  been 
made  use  of  in  conjunction  with  the  current  of  oxy- 
gen to  effect  the  combustion  of  organic  substances. 
The  method  best  adapted  for  general  purposes  involves 
the  use  of  cupric  oxide  and  a  current  of  air  or  oxygen. 

In  this  last  method  all  volatile  organic  products  are 


2  ELEMENTARY   ORGANIC    ANALYSIS 

oxidized  by  a  layer  of  hot  cupric  oxide ;  and  air,  or 
better,  oxygen  is  finally  introduced  to  aid  in  oxidizing 
any  charred  residue  and  to  reoxidize  the  reduced  cop- 
per. Of  the  products  formed,  water  is  absorbed  by 
calcium  chloride  or  sulphuric  acid ;  carbon  dioxide  is 
absorbed  by  a  solution  of  potassium  hydroxide  or  by 
soda-lime ;  and  the  nitrogen,  if  present,  escapes  as  such 
from  the  tube. 

PREPARATION  OF  OXYGEN 

The  commonest  method  of  preparing  oxygen  is  by 
heating  a  mixture  of  potassium  chlorate  and  manganese 
dioxide.  The  two  ingredients  should  be  separately 
pulverized  and,  if  necessary,  dried.  Equal  weights  of 
each  are  intimately  mixed  and  heated  in  a  250  cc. 
Jena  glass  Erlenmeyer  flask,  fitted  with  a  stopper  and 
a  wide  glass  elbow,  and  clamped  on  a  retort  stand.  To 
the  elbow  is  attached  a  rubber  tube  with  a  glass  elbow 
at  the  other  end  which  permits  a  free  movement  of  the 
tube  in  the  pneumatic  trough  or  gasometer.  On  grad- 
ually increasing  the  heat  a  rapid  though  steady  evo- 
lution of  oxygen  is  obtained.  At  the  end  of  the  re- 
action and  before  the  lamp  is  removed  the  tube  must 
be  withdrawn  from  the  pneumatic  trough  or  gasome- 
ter to  prevent  the  back  suction  of  the  water.  If  glass 
other  than  Jena  is  used  the  flask  should  be  protected 
in  heating  with  a  piece  of  asbestos  paper. 

The  purity  of  the  manganese  dioxide  is  of  consider- 
able importance  as  an  admixture  of  carbonaceous  mat- 
ter of  any  nature  is  liable  to  cause  an  explosion,  be- 


COMPRESSED   OXYGEN  3 

sides  contaminating  the  oxygen  with  carbon  dioxide. 
It  is  accordingly  advisable  to  test  a  small  portion  of 
the  mixture  by  heating  it  in  a  test-tube. 

COMPRESSED  OXYGEN 

The  use  of  compressed  oxygen  for  elementary  or- 
ganic analysis  is  especially  to  be  recommended  as  fur- 
nishing a  constant,  ready  supply  of  the  gas.1  It  is  fre- 
quently necessary  to  refill  a  gasometer  during  a  com- 
bustion, particularly  if  there  is  any  considerable  leak- 
age, and  it  is  not  always  possible  to  prepare  the  gas 
quickly  according  to  any  of  the  methods  ordinarily  in 
use.  A  cylinder  of  the  compressed  gas  can  be  used  at 
any  time  to  refill  the  gasometer  or,  what  is  still  more 
advantageous,  to  supply  the  oxygen  to  the  tube  with- 
out the  use  of  a  gasometer. 

Progress  in  the  compressed  gas  industry  has  reached 
such  a  point  that  it  is  comparatively  easy  to  secure  a 
strong,  thoroughly  well-tested  cylinder  of  highly-com- 
pressed oxygen.  These  cylinders  are  made  in  all  sizes, 
containing  from  a  few  gallons  to  a  hundred  cubic  feet 
of  oxygen.  For  the  laboratory,  a  cylinder  containing 
ten  cubic  feet  or  seventy-five  gallons  is  of  convenient 
size. 

The  gas  thus  compressed  contains  slight  traces  of 
carbon  dioxide,  water  vapor,  and  nitrogen,  but  the 
most  vexatious  impurities  existing  in  compressed  oxy- 
gen are  volatile  hydrocarbons  resulting  from  the  su- 
perheating of  the  oils  used  in  lubricating  the  com- 

1  J.  Am.  Chem.  Soc.,  21,  389. 


4  ELEMENTARY   ORGANIC   ANALYSIS 

pressing  machinery.  With  compressed  oxygen  con- 
taining these  impurities  a  preheating  furnace1  is  neces- 
sary to  oxidize  the  hydrocarbons. 

Some  manufacturers  have  replaced  the  lubricating 
oils  with  graphite  and  in  the  large  number  of  cylin- 
ders of  compressed  oxygen  obtained  from  the  S.  S. 
White  Dental  Mfg.  Co.,  of  Princes  Bay,  N.  Y.,  and 
tested  in  this  laboratory,  gaseous  hydrocarbons,  or  at 
least  hydrocarbons  not  completely  absorbed  by  the 
sulphuric  acid  of  the  drying  apparatus,2  were  never 
detected. 

Only  compressed  oxygen  free  from  hydrocarbons  is 
recommended  as  the  preheating  furnace  is  an  unde- 
sirable addition  to  an  already  elaborate  apparatus.  The 
gas  obtained  from  the  above-mentioned  source  is  com- 
paratively dry  and,  after  being  freed  from  traces  of 
moisture  and  carbon  dioxide,  is  sufficiently  pure  to  be 
used  in  elementary  organic  analyses. 

A  cylinder  containing  ten  cubic  feet  costs  about  ten 
dollars.  The  so-called  "commercial"  oxygen,  which 
differs  from  the  "medical "  only  in  so  far  as  it  has  been, 
perhaps,  a  little  less  thoroughly  washed  and  purified, 
costs  at  the  rate  of  ten  cents  per  foot;  i.  e.,  one  dollar 
per  ten  cubic  feet.  This  amount  is  not  great  com- 
pared with  the  cost  of  an  ordinary  Mitscherlich  or 
Pepy  gasometer.  Inasmuch  as  the  gasometers  are  used 
almost  exclusively  to  hold  oxygen,  it  will  be  seen  that 
they  are  not  indispensable  to  the  ordinary  laboratory 
supplied  with  a  cylinder  of  the  gas.  Ten  cubic  feet 

1  J.  Am.  Chem.  Soc.,  15,  531. 
*  See  page  14. 


COMPRESSED    OXYGEN 


will  last  for  a  great  many  carbon  and  hydrogen  com- 
bustions, while  the  advantage  of  using  a  cylinder  of 
this  gas  in  the  lecture  room  is  obvious. 

Inasmuch  as  the  oxygen  contained  in  the  steel  cyl- 
inders is  under  great  pressure,  some  method  of  regu- 
lating the  flow  of  gas  as  it  en-  ^ 
ters  the  combustion  tube  must 
be  devised.  The  expensive  pre- 
cision and  reduction  valves  used 
ordinarily  with  these  cylinders 
can  be  replaced  by  the  follow- 
ing  simple    arrangement.      A 
rubber  tube  leading  from  the 
cylinder  connects  with  a  T-tube, 
one  end  of  which  dips  one  inch 
under  mercury  in  a  small  bot- 
tle fitted  with  a  rubber  stopper 
having  two  holes  (Fig.  i).    The 
second  hole   is   left   open.     A 
rubber  tube  connects  the  other 
end  of  the  T-tube  with  the  puri- 
fying   apparatus,   which  is    in 
turn     connected     by      rubber 
tubing  to  the  combustion  tube. 
This  last  rubber  tube  is  sup- 
plied with  a  pinch-cock  which                 Fig-  i- 
is   closed.       When    the   valve  .  on    the   oxygen   cyl- 
inder is  opened   slightly   the  gas  will  flow  out,  and, 
passing  through  the  trap,  produce  a  bubbling  in  the 
mercury.     As  yet  no  gas  escapes  through  the  purify- 


ELEMENTARY   ORGANIC   ANALYSIS 


ing  apparatus.  If,  now,  one  wishes  to  regulate  the 
flow  of  gas,  the  pinch-cock  is  opened  until  the  desired 
rate  of  flow  is  secured.  Any  excess  of  gas  still  escapes 
through  the  mercury  trap.  It  is  always  possible  to 
adjust  the  valve  on  the  oxygen  cylinder  finally,  so  as 
to  prevent  any  appreciable  loss  of  oxygen  through  the 
mercury. 

As  the  weight  of  the  cylinder,  as  well  as  that  of  the 
oxygen  it  contains,  is  generally  given,  by  keeping  a 
record  of  the  weight  of  the  cylinder  it  is  easy  at  any 
time  to  determine  how  much  oxygen  remains.  If  a 
high  pressure  gauge  is  at  hand  a  similar  result  may 
be  obtained  by  recording  the  diminution  in  pressure.1 

GASOMETERS  OR  GAS-HOLDERS 

Where  the  air  or  oxygen  used  is  not  under  pressure 
L  in  cylinders  or  pipes  connected 

with  a  constant  supply,  some 
form  of  gasometer  is  necessary. 
A  Pepy  gasometer  (Fig.  2)  con- 
structed of  zinc  or  preferably 
of  copper  is  less  liable  to  be 
broken  than  the  Mitscherlich 
or  glass  form  though  there  is 
great  advantage  in  being  able 
to  see  at  a  glance  the  amount 
of  gas  remaining  in  the  appara- 
tus. For  elementary  analysis 
alone  the  Pepy  form  has  one 
Fig.  2.  unnecessary  pipe  and  stop-cock 

1  Miiller:  Ztschr.  physik.-chem.  Unterricht,  13,  26. 


GASOMETERS   OR   GAS-HOLDERS  7 

(C)   which  is  used  to  direct  gas  into  cylinders  filled 
with  water  and  inverted  in  the  upper  vessel. 

The  gasometer  is  filled  with  water  by  closing  the 
opening  near  the  bottom  and  opening  all  valves  at  the 
top  and  then  conducting  into  the  upper  pan  a  current 
of  water  which  flows  through  the  connecting  pipes  and 
soon  fills  the  lower  chamber.  The  rise  of  the  water 
is  noted  by  means  of  the  side  gauge  tube.  When  the 
gasometer  is  completely  filled  all  the  cocks  are  tightly 
closed  and  the  screw  plug  or  cock  near  the  bottom 
removed.  A  little  water  will  run  out,  especially  if 
the  chamber  has  not  been  completely  filled  with  water. 
After  the  first  few  cubic  centimeters  have  escaped  no 
more  water  should  run  out  and  gas  from  a  generator 
or  cylinder  may  be  conducted  by  means  of  a  bent  tube 
through  the  opening  in  such  a  manner  that  it  rises 
inside  the  chamber,  the  displaced  water  running  out 
at  the  opening.  Consequently  it  is  necessary  in  filling 
a  gasometer  of  this  form  to  conduct  the  operation  at  a: 
sink  or  to  provide  in  some  other  way  for  the  displaced 
water.  When  filled  with  gas  the  lower  chamber  is 
tightly  closed  and  the  upper  pan  is  filled  with  water. 
On  opening  the  stop-cock  A,  the  water  in  the  pan  flows 
down  through  a  tube  extending  to  the  bottom  of  the 
lower  chamber  until  the  internal  pressure  of  the  gas 
is  sufficient  to  sustain  the  column  of  water  in  the  tube. 
By  carefully  opening  stop-cock  B,  gas  may  be  with- 
drawn as  desired.  As  gas  is  withdrawn  the  water 
flows  through  the  valve  A  into  the  lower  chamber  and 
provision  must  be  made  to  furnish  occasionally  a  sup- 


8  ELEMENTARY   ORGANIC   ANALYSIS 

ply  of  water  for  the  upper  pan  as  otherwise  as  soon  as 
all  the  water  in  the  pan  has  run  into  the  lower  cham- 
ber, the  flow  of  gas  would  cease.  Instead  of  intermit- 
tently adding  water  to  the  upper  pan  a  simple  device 
may  be  used  for  maintaining  a  constant  level  of  water 
in  the  pan.  This  consists  of  a  rubber  tube  conduct- 
ing water  from  a  tap  through  a  twenty  centimeter 
length  of  small  lead  pipe,  bent  in  the  form  of  a  U  and 
hung  over  the  edge  of  the  pan.  A  hole  is  made  in  the 
side  of  the  pan  about  half  way  down  and  a  cork  carry- 
ing a  tube  leading  to  the  sink  is  carefully  inserted. 
Water  flowing  through  the  lead  U  fills  the  pan  to  the 
level  of  the  opening  and  all  excess  of  water  flows  away 
through  the  overflow  to  the  sink. 

The  Mitscherlich  form  of  gasometer  is  manipulated  in  es- 
sentially the  same  manner,  the  operation  of  the  valves  differ- 
ing in  no  way  from  that  in  the  Pepy  form.  The  adoption  of 
the  device  for  maintaining  a  constant  level  in  the  upper  jar 
is  rendered  somewhat  difficult,  as  a  hole  is  not  as  readily 
made  through  glass  as  through  metal.  However,  any  one 
may,  with  a  broken  file  and  emery  and  camphor,  grind  a  hole 
of  suitable  size  through  the  glass.  By  slipping  a  piece  of 
thick-walled  rubber  tubing  over  the  glass  tube  used  as  an 
overflow  it  can  be  inserted  in  the  orifice  which  accordingly 
need  not  be  unnecessarily  large. 

The  use  of  gasometers  of  these  forms  is,  however, 
open  to  considerable  objection,  for  if  constructed  of 
metal  they  soon  leak,  owing  to  the  attacks  of  acid 
fumes  and  vapors  always  present  in  a  laboratory,  and 
are  constantly  needing  repairs,  and  glass  gasometers, 
though  they  do  away  with  the  large  mass  of  metal, 
rely  on  metallic  pipes,  cocks,  and  connections  to  re- 


GASOMETERS   OR   GAS-HOLDERS  9 

ceive  and  deliver  the  gas.  These  connections  are 
equally  liable  to  the  attack  of  acid,  resulting  in  leak- 
age and  consequent  loss  of  gas.  The  fragile  nature  of 
such  gasometers  and  the  necessity  for  transportation 
to  the  sink,  when  being  filled,  make  their  handling 
difficult.  Consequently  it  is  only  by  exercising  great 
care  in  the  lubrication  of  stop-cocks  and  in  the  general 
treatment  of  this  form  of  gas-holder  that  satisfactory 
service  can  be  obtained. 

A  much  simpler  though  less  convenient  form  of  gas-holder 
(Fig.  3)  consists  of  two  carboys,  demijohns,  or  large  bottles 


Fig.  3- 

fitted  with  two-holed  rubber  stoppers  through  one  hole  of 
which  a  glass  tube  extends  to  the  bottom  of  the  vessel.  The 
other  hole  in  one  of  the  stoppers  is  provided  with  a  glass  elbow 
carrying  a  short  piece  of  rubber  tubing  and  a  screw  cock.  A 
rubber  tube  once  and  a  half  as  long  as  the  vessel  is  high,  con- 
nects the  two  long  glass  tubes  in  the  two  vessels.  One  bot- 
tle is  completely  filled  with  water  and  the  screw  pinch-cock 
closed.  The  other  is  then  lowered  until  its  mouth  is  about 
on  a  level  with  the  bottom  of  the  bottle  containing  water. 


10  ELEMENTARY   ORGANIC   ANALYSIS 

The  delivery-tube  of  the  gas  generator  from  which  only  a 
gentle  current1  of  oxygen  must  be  escaping  is  connected  with 
the  short  rubber  tube,  the  screw-cock  having  been  previously 
opened,2  and  the  gas  entering  the  bottle  forces  the  water 
through  the  long  rubber  tube  into  the  other  bottle.  At  the 
end  of  the  operation  the  gas  generator  is  removed  and  the 
screw-cock  immediately  closed.  By  raising  the  bottle  con- 
taining water  to  a  position  somewhat  above  the  level  of  the 
bottle  containing  gas,  almost  any  degree  of  pressure  may  be 
obtained.  The  gas  is  drawn  off  by  slowly  opening  the  screw 
pinch-cock. 

Bottles  having  tubulatures  at  the  bottom  are  used  with  a 
one-holed  stopper  carrying,  in  the  one  case,  a  short  glass 
elbow  and  screw-cock,  while  the  hole  in  the  other  stopper  is 
left  open.  The  long  rubber  tube  connects  short  pieces  of 
glass  tube  thrust  through  stoppers  in  the  tubulatures.  The 
manipulation  differs  in  no  wise  from  that  given. 

Another  convenient  form  of  gasometer  is  a  modification  of 
the  foregoing,  consisting,  however,  of  only  one  vessel  fitted 
with  a  two-holed  rubber  stopper,  the  holes  of  which  are 
plugged  with  four  centimeter  lengths  of  glass  rod  or  glass 
tubing  sealed  at  one  end.  The  vessel  is  filled  with  gas  at  a 
pneumatic  trough  and  the  stopper  firmly  inserted  while  the 
neck  is  still  under  water.  The  vessel  is  then  placed  on  the 
table,  one  of  the  plugs  withdrawn  and  a  glass  elbow,  carry- 
ing a  short  piece  of  rubber  tube  and  a  screw-cock,  quickly 
thrust  into  the  hole  in  the  stopper.  A  glass  elbow  in  the 
end  of  a  rubber  tube  connected  to  a  tap  is  first  filled  with 

1  Determined  by  dipping  the  tube  for  a  moment  under  water  in  a  beaker 
and  noting  the  rate  of  bubbling. 

2  The  cock  is  opened  and  in  case  there  was  sufficient  water  in  the  long 
rubber  tube  to  cause  it  to  act  as  a  siphon  the  action  is  momentarily  stopped  by 
pinching  the  connecting  tube  with  the  fingers,  care  being  taken  to  release  the 
pressure  on  the  tube  as  soon  as  the  oxygen  generator  is  connected.    By  using 
care  in  filling  the  bottle  no  water  is  driven  over  into  the  rubber  tube  and  the 
siphon  is  not  started  until  the  pressure  of  the  gas  in  the  generating  apparatus 
forces  the  water  up  through  the  bend  of  the  long  glass  tube  into  the  rubber 
connecting  tube  whence  it  falls  into  the  other  bottle. 


AIR  II 

water  by  allowing  the  water  to  flow  through  the  tube  for  a 
moment,  and  then  thrust  into  the  second  hole  of  the  stopper 
after  removing  the  plug.  The  connection  is  made  between 
the  rubber  tube  and  the  purifying  apparatus  and  the  screw- 
cock  is  opened.  By  admitting  water  through  the  glass  elbow, 
gas  in  desired  quantities  may  be  forced  through  the  system. 
If  a  three-holed  stopper  is  used  and  a  two  or  three  centimeter 
layer  of  water  is  left  in  the  bottom  of  the  vessel  a  long 
glass  tube  may  be  thrust  through  the  third  hole  far  enough 
to  dip  under  water  in  the  bottom  and  thus  serve  as  a  pressure 
gauge. 

A  fundamental  objection  to  all  forms  of  gasometers 
is  the  great  inconvenience  experienced  in  filling  them 
at  a  sink  with  gas  or  water,  requiring,  as  they  do, 
transportation  of  a  bulky  apparatus  filled  with  water. 
Accordingly  whenever  obtainable  the  use  of  pure  com- 
pressed oxygen  in  steel  cylinders  is  strongly  recom- 
mended. 

AIR 

In  some  methods  of  analysis  air  is  substituted  for 
oxygen  during  the  greater  part  of  the  combustion, 
oxygen  being  used  only  to  oxidize  refractory  carbona- 
ceous residues.  The  use  of  air  in  gasometers  is  iden- 
tical with  the  manipulation  described  on  page  7  with 
the  exception  that  the  filling  is  accomplished  by  open- 
ing the  lower  seal  and  the  valve  from  which  the  gas 
is  usually  drawn  at  the  top.  For  most  organic  bodies 
oxygen  must  be  substituted  for  air  before  complete 
oxidation  can  be  assured,  hence  two  aspirators  and  puri- 
fiers are  often  used. 

Air  under  pressure  may  also  be  conveniently  ob- 
tained from  any  of  the  numerous  forms  of  water-blast. 


12  ELEMENTARY   ORGANIC   ANALYSIS 

PURIFYING  APPARATUS 

Impurities  other  than  carbon  dioxide  and  water- 
vapor  are  seldom  present  in  oxygen  or  air  used  for 
analysis,  though  in  the  compressed  oxygen  furnished 
by  some  manufacturers  material  quantities  of  hydro- 
carbons are  often  present.  These  hydrocarbons  are 
effectually  removed  by  conducting  the  gas  direct  from 
the  gasometer  or  cylinder  through  a  heated  glass,  por- 
celain,1 or  brass2  tube  containing  cupric  oxide  where 
they  are  oxidized  to  water  and  carbon  dioxide.  In 
general  the  removal  of  carbon  dioxide  and  water-vapor 
only  is  necessary. 

The  removal  of  water-vapor  is  effected  by  use  of 
calcium  chloride  or  concentrated  sulphuric  acid,  while 
potassium  or  sodium  hydroxide  in  sticks  or  in  con- 
centrated solution,  or  soda-lime  is  used  to  remove  all 
traces  of  carbon  dioxide.  The  relative  merits  of  the 
various  absorbing  agents  are  discussed  at  length  under 
the  head  of  absorbing  agents  on  page  31. 

The  essential  feature  of  the  purifying  system  is  that 
it  should  hold  a  considerable  quantity  of  the  reagents 
and  not  become  exhausted  until  after  a  large  number 
of  combustions.  The  gas  entering  the  purifying  ap- 
paratus is  generally  very  moist  from  standing  over 
water  in  a  gasometer  and  contains  but  a  small  quan- 
tity of  carbon  dioxide.  The  gas  leaving  the  combus- 
tion tube  contains  as  a  rule  even  more  water  and  a 
great  deal  more  carbon  dioxide,  so  much  more  in  fact 

1  Dudley  and  Pease  :  J.  Am.  Chem.  Soc.,  15,  530. 

2  Shimer:  lbid.>  ai,  560. 


PURIFYING   APPARATUS  13 

that  the  absorbing  reagents  ordinarily  iised  become 
exhausted  after  one  or  two  combustions. 

Of  the  various  absorbents  of  carbon  dioxide  and 
water  the  systems  most  commonly  used  are :  Potas- 
sium hydroxide  followed  by  calcium  chloride  ;  potas- 
sium hydroxide  followed  by  sulphuric  acid ;  sulphuric 
acid,  potassium  hydroxide,  and  sulphuric  acid  ;  sul- 
phuric acid,  potassium  hydroxide,  and  calcium  chlo- 
ride, etc.,  etc.,  and  sulphuric  acid,  soda-lime,  and  sul- 
phuric acid. 

The  last  combination  is  remarkably  efficient  and 
hence  is  first  considered. 

A  simple  form  of  purifying  apparatus  consists  of  a 
Drechsel  gas  washing-bottle  one-third  filled  with  con- 
centrated sulphuric  acid,  a  U-tube  containing  soda- 
lime,1  and  a  U-tube  containing  pumice  stone  drenched 
with  concentrated  sulphuric  acid.  The  sulphuric  acid 
in  the  bottle  retains  the  water  (in  case  a  cylinder  of 
compressed  oxygen,  which  is  itself  very  dry,  is  used, 
the  gas  washing-bottle  with  sulphuric  acid  may  be 
discarded),  the  soda-lime  retains  the  carbon  dioxide,  and 
the  sulphuric  acid  and  pumice  stone  U-tube  the  moisture 
escaping  from  the  soda-lime.  The  gas  issuing  from 
this  system  is  free  from  carbon  dioxide  and  contains 
no  moisture  that  can  be  retained  by  sulphuric  acid. 

The  three  pieces  of  the  absorber  above  described 
may  be  combined  by  using  a  calcium  chloride  jar  filled 
as  is  shown  in  Fig.  4.2  The  tubulature  should  be  as 

1  Page  32. 

2  Am.  Chem  .  J.,  23,  332. 


ELEMENTARY   ORGANIC    ANALYSIS 


near  the  top  of  the  lower  compartment1  as  possible,  to 
permit  the  introduction  of  the  maxi- 
mum quantity  of  concentrated  sul- 
phuric acid. 

A  piece  of  glass  tubing  of  an  ex- 
ternal diameter  a  little  less  than  the 
internal  diameter  of  the  constriction 
in  the  jar  is  cut  off  long  enough  to 
rest  on  the  bottom  and  reach  within 
thirty  millimeters  of  the  top  of  the 
jar.  A  one-holed  cork  on  the  end 
of  a  glass  rod  is  loosely  inserted  in 
the  upper  end  of  the  tube  and  a  layer 
of  glass  wool  or  long  fiber  asbestos 
is  packed  around  the  tube  to  a  depth 
of  three  or  four  millimeters.  Soda- 
lime  prepared  as  described  on  page 
32  and  pulverized  into  pieces  ap- 
proximately two  millimeters  in  di- 
ameter is  then  introduced  and  the  jar 

filled  to  within  one  centimeter  of  the  top  of  the  inner 

1  Calcium  chloride  jars  are  furnished  by  Whitall,  Tatum  &  Co.  with  the 
tubulature  inserted  at  any  point  desired  in  the  lower  compartment  at  a  cost 
but  slightly  in  advance  of  the  regular  goods.  As  ordinarily  made  a  jar  "  12 
inches  high"  will  have  the  tubulature  in  such  a  position  that  about  20  cc.  of 
concentrated  sulphuric  acid  can  be  introduced  without  flowing  out  of  the  orifice 
or  coming  in  contact  with  a  rubber  stopper  inserted  in  the  tubulature.  Where 
gas  from  a  cylinder  of  compressed  oxygen  is  used  this  amount  of  acid  suffices 
to  dry  a  great  many  liters  of  gas,  but  if  the  gas  is  drawn  from  gasometers  over 
water  the  reagent  is  more  rapidly  exhausted.  Twenty  cc.  of  acid  will  absorb 
effectually  six  grams  of  water  and  taking  the  temperature  of  a  gasometer  in  use 
as  28°  C.,  the  gas  leaving  it  would  contain  twenty-seven  milligrams  of  water- 
vapor  per  liter.  An  average  of  1.5  liters  of  oxygen  are  used  per  combustion  in 
the  method  described  beyond,  hence  twenty  cc.  of  acid  would  serve  for  two 
hundred  combustions,  a  number  rarely  exceeded  by  any  one  operator.  The 
chief  advantage  of  that  form  of  jar  having  the  tubulature  near  the  top  of  the 
base  is  that  there  is  much  less  liability  of  getting  concentrated  acid  on  the 
rubber  stopper  in  the  tubulature. 


Fig.  4. 


PURIFYING    APPARATUS  15 

tube.  A  "  1 2-inch"  calcium  chloride  jar  will  require 
about  one  hundred  and  seventy-five  grams  of  soda-lime. 
The  layer  of  glass  wool  or  asbestos  prevents  the  soda- 
lime  from  falling  through  into  the  lower  compartment. 

A  long  glass  tube  approximately  ten  millimeters 
external  diameter  is  thrust  through  the  one-hole  rub- 
ber stopper  inserted  in  the  top  of  the  jar.  This  tube 
is  slightly  constricted  at  the  bottom  and  is  filled  with 
pumice  stone  which  is  subsequently  drenched  with 
concentrated  sulphuric  acid.  The  glass  tube  extends 
from  about  one  centimeter  above  the  cork  to  within 
two  centimeters  of  the  bottom  and  should  be  of  a  diam- 
eter small  enough  to  slide  easily  through  the  upright 
tube  passing  through  the  constriction  of  the  jar.  The 
upper  end  of  the  tube  is  closed  with  a  one-holed  red 
rubber  stopper  carrying  a  glass  elbow  and  a  piece  of 
rubber  tubing  with  a  screw  pinch-cock.  This  stopper 
may  be  sealed  with  paraffin  if  desired.  With  this  ar- 
rangement the  only  chance  for  leakage  that  could  con- 
taminate the  gas  is  in  the  stopper  at  the  top  of  the 
glass  tube.  A  leak  at  the  other  stoppers  would  be 
effectively  counteracted  so  far  as  moisture  (the  great- 
est danger  in  leaks)  is  concerned  by  the  long  column 
of  pumice  stone  and  sulphuric  acid. 

Concentrated  sulphuric  acid  is  poured  down  the 
central  tube  thoroughly  drenching  the  pumice  stone 
and  collecting  in  the  base.  The  first  lot  of  acid  is 
often  contaminated  with  foreign  material  from  the 
pumice  stone  and  should  be  poured  out  of  the  tubula- 
ture;  enough  acid  is  then  poured  through  the  tube  to 


1 6  ELEMENTARY   ORGANIC    ANALYSIS 

fill  the  lower  compartment  to  within  five  millimeters 
of  the  one-holed  rubber  stopper  in  the  tubulature.  The 
cork  is  then  replaced  in  the  tube  and  the  pinch-cock 
closed.  A  glass  tube  bent  downwards  is  thrust  through 
the  hole  in  the  rubber  stopper  in  the  tubulature  in 
such  a  manner  that  a  current  of  gas  passed  through  it 
bubbles  through  the  acid  in  the  base  of  the  jar.  The  gas 
rises,  passes  through  a  long  column  of  soda-lime  at  a  very 
slow  rate,  and  then  turns  and  passes  down  through  the 
annular  space  between  the  two  glass  tubes,  finally  en- 
tering the  base  of  the  tube  filled  with  pumice  stone 
and  issuing  at  the  top.  The  greater  portion  of  the 
water  is  removed  as  the  gas  bubbles  through  the  acid ; 
the  carbon  dioxide  is  completely  removed  by  the  soda- 
lime,  and  the  unabsorbed  moisture,  including  that  lost 
from  the  slightly  moist  soda-lime,  is  removed  as  the 
gas  passes  over  the  pumice  stone  and  sulphuric  acid. 
The  gas  issuing  at  the  top  fs  free  from  carbon  dioxide 
and  as  free  from  moisture  as  is  possible  with  sulphuric 
acid.  In  case  the  sulphuric  acid  becomes  exhausted, 
to  regenerate  the  purifier  it  is  only  necessary  to  drain 
the  acid  out  of  the  lower  compartment  and  pour  fresh 
acid  through  the  tube  containing  the  pumice  stone. 
This  operation  should  be  performed  at  the  end  of  every 
fifty  combustions.  The  soda-lime  need  not  be  renewed 
until  it  becomes  three-fourths  white. 

Numerous  forms  of  apparatus,  more  or  less  complex,  for 
holding  the  various  absorbents  for  the  purification  of  the  air 
or  oxygen  used  in  organic  combustions,  are  described  in 
chemical  literature  and  are  furnished  by  most  of  the  dealers 
in  chemical  supplies.  Some  are  suited  for  the  absorbing  com- 


RUBBER   TUBING   AND   STOPPERS  1 7 

bination  sulphuric  acid,  soda-lime,  sulphuric  acid  and  many 
were  especially  devised  and  are  described  as  being  filled  with 
the  other  absorbents  in  any  of  the  other  combinations.  Liquid 
reagents,  such  as  potassium  hydroxide  solution  or  concen- 
trated sulphuric  acid,  are  generally  held  in  gas  washing-bot- 
tles though  sometimes  they  are  poured  over  broken  pumice 
stone  in  long  U-tubes.  Solid  reagents,  such  as  calcium  chlo- 
ride, stick  potassium  hydroxide,  and  soda-lime,  are  generally 
placed  in  large  U-tubes  or  calcium  chloride  jars.  The  re- 
markable absorptive  power  of  soda-lime  for  carbon  dioxide 
renders  it  unnecessary  to  have  a  great  length  of  this  reagent 
through  which  the  gas  must  pass. 

RUBBER  TUBING  AND  STOPPERS 

Rubber  tubing  is  used  to  make  all  connections  be- 
tween the  oxygen  supply,  the  purifier,  and  the  com- 
bustion tube,  and  only  fresh  tubing,  not  cracked  or 
deteriorated,  must  be  used.  Often  strong  sulphuric 
acid  is  carelessly  allowed  to  suck  back  into  the  rubber 
tube  connecting  the  supply  of  oxygen  with  the  puri- 
fier. The  tubing  becomes  hard  and  brittle  and  should 
immediately  be  removed. 

Red  or  maroon  tubing,  "5/32  inch "  inside  diame- 
ter, is  a  convenient  size  and  withstands  the  action  of 
heat  better  than  most  other  kinds.  A  twenty  to  forty 
centimeter  length  is  required  to  conduct  the  gas  from 
the  purifier  to  the  combustion  tube,  but  the  gas  issuing 
from  the  combustion  tube  does  not  come  in  contact 
with  any  length  of  tubing  as  it  is  advisable  to  have 
the  glass  tubes  of  the  various  absorbers  touch,1  using 
a  two  centimeter  length  of  rubber  tube  simply  to  hold 

1  Pfliiger  :  Arch.  f.  d.  ges.  Physiol.,  18,  133  ;  lyieben,  cited  in  "  Manual  of  Or " 
ganic  Chemistry, "  by  L,assar-Cohn,  p.  373  ;  Berthelot  :  Compt.  rend.,  no,  684. 
2 


1 8  ELEMENTARY   ORGANIC   ANALYSIS 

the  glass  tubes  together.     If  glass  touches   glass   a 
minimum  exposure  to  rubber  is  obtained. 

The  rubber  stoppers  used  in  the  purifier  and  in  the 
absorbing  tubes  may  be  of  the  ordinary  quality  though 
red  rubber  is  preferable.  Those  used  in  the  ends  of 
the  combustion  tube  ought  to  be  of  red  rubber  as  it  is 
much  less  affected  by  heat  than  any  other  kind. 

COMBUSTION  FURNACES 

The  accuracy  demanded  in  organic  elementary  analy- 
sis at  the  present  day  renders  the  use  of  a  gas  com- 
bustion furnace  almost  imperative.  The  earlier  forms 
of  charcoal  furnace  are  occasionally  used  in  localities 
where  gas  is  not  at  hand,  but  they  are  not  of  sufficient 
importance  to  warrant  special  consideration  here. 

Where  illuminating  gas  is  accessible  any  one  of  the 
numerous  forms  of  furnace  may  be  advantageously 
used.  In  general  the  furnace,  which  is  about  eighty 
centimeters  long,  consists  of  a  series  of  twenty  to  thirty 
Bunsen  burners  provided  with  air  as  well  as  gas  regu- 
lating devices,  screwed  into  a  gas  pipe  of  size  large 
enough  to  supply  all  the  burners  with  sufficient  gas, 
giving  a  heating  length  of  about  seventy-five  centi- 
meters. The  flames  are  generally  confined  and  made 
to  impinge  on  fire  clay  tiles  in  such  a  manner  that  the 
combustion  tube  is  heated  on  all  sides  as  evenly  as 
possible.  A  good  pressure  as  well  as  a  good  supply 
of  gas  is  necessary  and,  owing  to  the  great  heat  and 
the  large  quantity  of  the  products  of  combustion,  the 
furnace  should  be  installed  in  a  hood  provided  with  a 


COMBUSTION   FURNACES  19 

good  draft.  Sufficient  room  for  the  gasometer  or  oxy- 
gen cylinder  and  the  purifying  apparatus  as  well  as 
the  absorbing  system  is  necessary.  A  gas  cock  to 
which  a  Bunsen  burner  with  a  long  rubber  tube  is 
attached  should  be  at  hand.  The  gas  pipe  with  which 
the  furnace  is  connected  should  be  not  less  than  "  y±- 
inch"  standard  size. 

Many  designs  of  furnaces  are  in  the  market,  each 
constructed  with  a  view  of  attaining  the  greatest  and 
most  even  heating  of  the  combustion  tube  with  the 
minimum  consumption  of  gas.  The  progress  made  in 
developing  the  combustion  furnace  is  not  as  great  as 
the  complex  nature  of  many  designs  would  indicate, 
and  there  is  very  little,  if  any,  choice  in  the  selection 
of  a  furnace.  All  are  a  great  improvement  on  the 
charcoal  furnace,  and  all  have  their  salient  points.  As 
a  rule  the  increase  in  complexity  of  construction  and 
in  cost  is  not  accompanied  by  a  proportionate  increase 
in  utility.  Equally  satisfactory  results  with  the  "  Bun- 
sen,"  "  Erlenmeyer-Babo,"  and  u  Glaser  "  furnaces  have 
been  obtained  in  this  laboratory. 

The  ' '  Glaser ' '  T  furnace  is  provided  with  a  series  of  iron  U 
pieces  placed  side  by  side,  forming  a  trough  in  which  the 
combustion  tube  is  placed.  Difficulty  is  often  experienced, 
however,  as  the  pieces  become  separated  and  allow  the  free 
flame  to  impinge  on  the  unprotected  glass,  a  fracture  of  the 
tube  being  almost  certain  to  result  at  the  point  of  overheat- 
ing. If  the  tube  is  well  protected  at  the  bottom  with  two  or 
three  layers  of  asbestos  paper  the  liability  to  accident  is  much 
decreased. 

1  Ann.  Chem.  (I^iebig)  Suppl.,  7,  215. 


20  ELEMENTARY   ORGANIC   ANALYSIS 

In  most  other  forms  of  furnace,  including  the  two 
others  above  mentioned,  the  combustion  tube  is  laid 
in  a  heavy  sheet  iron  or  tile  trough. 

Tiles  possess  the  advantage  of  not  becoming  twisted 
or  warped  on  heating,  but  owing  to  their  fragile  nature 
and  their  non-conductivity  for  heat,  thus  necessitating 
considerable  time  to  heat  and  cool,  they  are  not  to  be 
recommended. 

Iron  troughs  are  open  to  the  serious  objection  that 
they  warp  and  twist  and  constantly  form  scales  of  iron 
oxide  which  clog  the  burners.  On  the  other  hand, 
they  are  very  cheap,  conduct  heat  much  better  than 
tile,  and  if  constructed  of  sheet  iron  two  millimeters 
in  thickness  do  not  warp  sufficiently  to  cause  any  con- 
siderable difficulty.  Any  plumber  or  tinner  can  make 
them  by  hammering  a  piece  of  thick  (two  millimeters) 
sheet  iron  over  a  gas  pipe.  They  may  be  renewed 
before  any  considerable  amount  of  scale  is  formed  and 
are  on  the  whole  the  most  satisfactory  form  of  trough 
to  use. 

The  trough,  whether  it  is  of  iron  or  tile,  should  be 
lined  with  two  or  three  layers  of  asbestos  paper  which 
will  protect  the  tube. 

A  fifteen  centimeter  square  of  rather  thick  asbestos 
card  having  a  two  centimeter  hole  in  the  center  is 
thrust  over  each  end  of  the  combustion  tube  as  a  pro- 
tection to  the  corks. 

The  consumption  of  gas  varies  not  only  with  the 
form  of  furnace,  but  more  especially  with  the  nature 
of  the  substance  to  be  burned.  A  number  of  experi- 


COMBUSTION   TUBKS  21 

ments  made  with  different  forms  of  furnaces  coupled 
direct  to  a  gas  meter  gave  an  average  consumption  of 
forty-five  cubic  feet  of  gas  per  hour. 

In  some  forms  of  furnace,  provision  is  made  for  rais- 
ing or  lowering  the  burners  which  is  often  an  advan- 
tage in  securing  the  best  regulation  of  the  flames.  The 
furnace  should  be  raised  some  three  or  four  centimeters 
at  the  anterior  end  by  placing  a  block  of  wood  of  the 
required  height  under  the  support.  (Fig.  12,  p.  51.) 

COMBUSTION  TUBES 

The  cupric  oxide  used  to  oxidize  the  volatile  por- 
tions of  the  material  to  be  burned  is  heated  in  a  plati- 
num, porcelain,  or  glass  tube.  Platinum  tubes  are 
expensive  and  consequently  but  rarely  used.  Porce- 
lain tubes  are  much  used  in  technical  work,  but  as  a 
rule  are  too  fragile  and  expensive  for  ordinary  labora- 
tory use.  Glass  tubes  are  almost  invariably  used  in 
all  other  than  technical  laboratories.  As  the  old  bayo- 
net form  of  tube  originally  used  by  Liebig  has  been 
almost  universally  replaced  by  the  tube  open  at  both 
ends,  the  former  need  not  be  considered  here.  The 
long-continued  high  heat  to  which  a  glass  combustion 
tube  is  subjected  requires  that  it  be  of  glass  of  unusual 
resistance  to  heat,  /.  £.,  not  melting  easily  and  capable 
of  withstanding  rapid  fluctuations  in  temperature. 

Bohemian  glass — An  especially  hard  glass  contain- 
ing potassium  instead  of  sodium,  i.  e.,  Bohemian  glass, 
has  been  used  for  this  purpose  with  satisfactory  results 
for  a  number  of  years.  It  is  drawn  in  tubing  of  a  con- 


22  ELEMENTARY    ORGANIC   ANALYSIS 

venient  size,  generally  from  twelve  to  fifteen  milli- 
meters internal  diameter,  designated  as  combustion 
tubing,  which,  unless  otherwise  ordered,  comes  in 
lengths  of  approximately  two  meters. 

Jena  glass — Recently  a  new  glass  of  a  remarkably 
high  melting-point  and  small  coefficient  of  expansion 
has  been  introduced  which  promises  to  prove  of  great 
value  in  organic  analysis.  A  special  kind  of  Jena 
glass,  made  expressly  for  combustion  tubing,  which 
must  not,  however,  be  confused  with  the  softer  glass 
used  for  preparing  sealed  tubes,  furnishes  a  tube  that 
leaves  very  little  to  be  desired  and  which  is  nearly  as 
resistant  to  heat  as  are  the  more  expensive  porcelain 
tubes. 

In  this  laboratory  the  average  number  of  combus- 
tions made  with  Jena  glass  tubes  is  over  fifty,  while 
four  tubes  have  withstood  heating  in  ninety-seven, 
ninety-nine,  one  hundred  and  four,  and  one  hundred 
and  sixteen  combustions  respectively. 

Owing  to  its  infusibility  there  is  much  less  danger 
of  breaking  Jena  glass  tubes  by  fusion  to  tile  or  iron 
troughs,  a  source  of  many  accidents  with  the  other 
forms  of  glass.  Fortunately  the  price  of  this  glass  is 
no  greater  than  that  of  other  kinds  of  combustion 
tubing. 

For  use  in  the  furnace  the  tube  should  be  cut  of  a 
proper  length  and  the  sharp  edges  should  be  rounded 
to  prevent  their  cutting  the  stopper  and  introducing 
bits  of  rubber  which  might  easily  be  swept  into  the 
heated  part  of  the  tube.  One  end  of  the  length  of 


COMBUSTION  TUBES  23 

combustion  tube  should  be  cut  off  square,  if  it  is  not 
already  so,  and  then  a  length  equal  to  the  length  of 
the  furnace  plus  ten  centimeters1  marked  off  with  a 
file  scratch  on  the  glass.  A  sharp  triangular  file  is 
necessary  for  the  purpose.  If  a  deep  scratch  is  made 
the  tube  may  readily  be  broken  at  the  scratch  in  case 
it  is  of  Bohemian  glass  by  following  a  line  around  the 
tube  in  the  direction  of  the  scratch  with  a  hot  glass 
rod,  or  better  with  a  gas  flame  burning  from  a  fine  jet. 
The  gas  flame  is  held  in  a  tangential  position  and  the 
tube  rapidly  revolved  three  or  four  times  to  produce 
excessive  heating  in  a  narrow  band  around  the  glass 
including  the  scratch.  Then  by  directing  the  flame 
on  the  scratch  for  a  few  moments  the  crack  will  usually 
start  and  instantly  run  clear  around  the  tube  in  the 
heated  zone  leaving  a  sharp  cut.  With  Jena  glass 
tubing  it  will  be  necessary  to  file  a  deep  scratch  clear 
around  the  tube.  On  applying  the  heat  from  a  fine 
gas  jet  as  described  the  tube  is  then  cut  with  no  great 
difficulty. 

After  cutting  the  tube  in  the  desired  length  the 
sharp  edges  of  the  cuts  are  rounded  off  either  by  filing 
with  a  round  file  or  better  by  fire-polishing,  i.  e.,  par- 
tially fusing  the  edges  in  a  blast-lamp.  The  fire-pol- 
ishing is  done  by  carefully  heating  one  end  of  the 
tube  in  a  smoky  flame  of  a  blast-lamp  until  the  glass 
is  covered  with  soot.  The  air  is  then  admitted  to  the 
blast-lamp  and  the  temperature  gradually  increased 

1  As  the  furnace  is  generally  eighty  centimeters  long  the  glass  tube  is  cut 
ninety  centimeters,  thereby  allowing  five  centimeters  to  protrude  at  each  end 
of  the  furnace. 


24  ELEMENTARY   ORGANIC   ANALYSIS 

till  the  edges  are  fused.  The  cooling  should  be  slow 
and  is  brought  about  by  cutting  off  the  air  from  the 
blast-lamp  and  gradually  turning  off  the  gas  till  the 
end  of  the  tube  is  again  covered  well  with  soot.  The 
flame  is  then  extinguished  and  the  tube  allowed  to 
cool  in  the  air.  The  covering  of  soot  equalizes  the 
radiation  of  heat  and  prevents  cracking. 

When  cool  the  tube  is  carefully  wiped  and  washed 
inside  and  outside  with  water  and  then  rinsed  with 
10-20  cc.  of  alcohol  which  is  allowed  to  drain  out. 
The  alcohol  adhering  to  the  walls  of  the  tube  is  re- 
moved by  rinsing  with  a  few  cubic  centimeters  of  ether 
and  the  tube  is  then  gently  warmed  by  moving  a  Bun- 
sen  burner  along  its  entire  length,  removing  the  ether- 
vapor  by  a  blast  of  air  or  by  suction  from  a  filter-pump. 

OXIDIZING  AGENTS  USED  IN  THE  COMBUSTION  TUBE 
Cupric  oxide.  — The  ease  with  which  this  compound 
gives  up  its  oxygen  to  reducing  substances  renders  it 
of  the  greatest  value  in  organic  combustions. 

For  general  purposes  the  wire  form,  consisting  of 
short  pieces  of  copper  wire  that  have  been  completely 
converted  to  the  oxide,  is  the  best.  This  form  is  readily 
obtained  in  the  market.  Approximately  four  hundred 
grams  of  the  oxide  in  the  wire  form  are  required  to 
fill  a  combustion  tube. 

Granular  copper  oxide  obtained  by  igniting  the  ni- 
trate is  often  used.  The  powdered  oxide  is  used  in 
small  quantities  to  cover  highly  refractory  substances. 
Each  time  before  being  used  it  should  be  heated 


OXIDIZING   AGENTS   USED   IN   COMBUSTION   TUBE     25 

strongly  in  a  Bunsen  flame  and  then  allowed  to  cool 
in  a  desiccator  where  it  should  be  preserved.  Both 
forms  are  readily  obtained  in  the  market. 

Spirals  of  copper  oxide  are  used  and  are  prepared 
by  winding  stout  copper  wire  tightly  around  a  glass 
tube  of  such  a  size  that  the  finished  spiral,  some  twelve 
centimeters  long,  can  easily  be  inserted  in  a  combus- 
tion tube.  The  wire  is  bent  in  a  ring  form  at  each 
end  to  facilitate  its  withdrawal.  The  spiral  is  then 
strongly  heated  in  a  Bunsen  flame  till  the  oil  is  all 
burned  off  and  an  outer  coating  of  black  cupric  oxide 
is  formed.  It  is  then  cooled  and  kept  in  a  desiccator 
until  used.  Another  form  of  spiral  which  is,  however, 
more  readily  disintegrated,  is  prepared  by  rolling  a 
piece  of  copper  gauze,  ten  centimeters  square,  around 
a  piece  of  stout  copper  wire  some  eleven  or  twelve 
centimeters  long,  with  a  loop  made  in  each  end.  The 
wire  gauze  is  compactly  rolled  and  the  roll  when 
finished  must  easily  slip  into  the  combustion  tube. 
The  fine  copper  wire  of  which  the  gauze  is  made,  read- 
ily oxidizes  when  heated  in  a  flame  which  burns  off 
any  shellac,  varnish,  or  oil.  The  brittle  nature  of  the 
cupric  oxide  renders  such  spirals  extremely  fragile  and 
they  soon  have  to  be  replaced.  Though  presenting 
less  surface  of  cupric  oxide  no  difficulty  will  be  experi- 
enced in  using  spirals  of  stout  wire  provided  they  are 
at  least  twelve  centimeters  long. 

Spirals  of  this  form  are  often  used  to  reduce  oxides  of  ni- 
trogen when  present  as  a  product  of  combustion  and  are  re- 
duced to  the  metallic  form  as  described  on  page  64. 


26  ELEMENTARY   ORGANIC   ANALYSIS 

A  succession  of  spirals1  has  been  used  in  place  of  the  wire 
form  of  cupric  oxide. 

Lead  chromate  is  the  only  other  oxidizing  agent 
that  is  extensively  used  in  ordinary  methods  of  com- 
bustion. This  material  in  the  fused  form  is  coarsely 
granulated  and  introduced  into  the  combustion  tube. 
It  is  used  in  burning  substances  containing  sulphur  or 
the  halogens  (p.  66).  No  special  preparation  is  neces- 
sary as  the  material  is  furnished  ready  for  use  in  the 
market. 

A  mixture  of  nine  parts  of  lead  chromate  and  one  part  of 
potassium  dichromate  was  introduced  by  Mayer,2  but  is  not 
generally  used. 

The  low  fusibility  of  lead  chromate  is  the  serious  disad- 
vantage in  its  use  and  to  obviate  this  difficulty  De  Roode3 
mixes  one  part  of  red  lead  with  four  parts  of  the  chromate. 

A  fused  mixture  of  lead  monoxide  and  cupric  oxide  was 
successfully  used  by  Schwarz.4 

Platinized  asbestos5  and  quartz,6  asbestos  covered  with 
finely  divided  cupric  oxide,7  manganese  sesquioxide,8  mer- 
curic oxide, 9  potassium  dichromate, I0  and  potassium  chlorate, " 
are  occasionally  used  but  belong  rather  to  special  methods. 

Other  oxidizing  agents  that  are  occasionally  used  to  cover 
refractory  substances  in  a  boat,  are  potassium  dichromate, 
potassium  chlorate,  platinum  sponge,  and  powdered  cupric 
oxide. 

1  Blau:  Monatshefte,  10,  357. 

2  Ann.  Chem.  (I,iebig),  95,  204. 

3  Am.  Chem.  J.,  12,  226. 

4  Ber.  d.  chem.  Ges.,  13,  566. 

5  Kopfer  :  Ztschr.  anal.  Chem.,  17,  4. 

6  Dennstedt:  "Die  Entwicklung  der  organischen  JJlementaranalyse,"  p.  103. 

7  lyippmann  and  Fleissner  :  Monatshefte,  7,  9. 

8  Dudley  :  Am.  Chem.  J.,  10,  433 ;  Ber.  d.  chem.  Ges.,  i,  3172. 

9  Mitscherlich  :  Ztschr.  anal.  Chem.,  15,  374;  Frerichs :   Ber.  d.  chem.  Ges., 
io,  26. 

10  Johnson  and  Hawes  :  Am.  J.  Sci.,  7,  465. 

11  Schulze  :  Ztschr.  anal.  Chem.,  5,  269. 


FILLING   THE   COMBUSTION  TUBE  27 

FILLING  THE  COMBUSTION  TUBE 

The  tube  cut  to  length,  cleaned  and  dried  as  de- 
scribed on  page  22,  is  filled  by  first  inserting  either  a 
plug  of  previously  ignited  fibrous  asbestos  or  a  short 
copper  oxide  spiral  five  centimeters  from  one  end  of 
the  tube  (Fig.  5). 

1          ••((•(((I .« f 

'  Fig-  5- 

The  asbestos  is  best  introduced  by  using  a  plunger 
consisting  of  a  glass  rod  about  ten  centimeters  long 
on  the  end  of  which  is  fastened  a  cork  whose  largest 
diameter  is  a  little  smaller  than  the  internal  diameter 
of  the  tube.  The  cork  is  inserted  in  the  tube  a  dis- 
tance of  five  centimeters,  crowding  before  it  a  small 
wad  of  asbestos.  Holding  the  glass  rod  and  the  com- 
bustion tube  firmly  in  a  vertical  position  and  keep- 
ing the  cork  five  centimeters  in  the  tube,  the  asbestos 
is  packed  down  by  a  similar  plunger  with  a  long  glass 
rod  or  tube  inserted  in  the  other  end  of  the  combus- 
tion tube.  More  asbestos  is  added,  if  necessary,  till  a 
plug  eight  to  ten  millimeters  long  is  obtained. 

If  cupric  oxide  spirals  are  used  they  must  be  prepared  by 
rolling  a  strip  of  copper  gauze,  one  centimeter  wide  and  ten 
centimeters  long,  into  a  roll  that  will  snugly  fit  the  interior 
of  the  combustion  tube.  The  roll  should  be  thoroughly  ig- 
nited in  a  Bunsen  flame  before  being  used . 

To  the  asbestos  plugs  there  is  the  possible  objection  that  a 
little  longer  time  is  required  to  oxidize  all  products  of  dry 
distillation  that  may  be  deposited  on  them  in  the  course  of  a 
combustion,  while  with  the  cupric  oxide  spirals  or  rolls  the 


28  ELEMENTARY   ORGANIC    ANALYSIS 

oxidation  of  such  material  would  be  effected  almost  imme- 
diately. On  the  other  hand  the  cupric  oxide  spirals  are 
easity  disintegrated.  Asbestos  plugs  have  been  exclusively 
used  in  this  laboratory. 

A  forty-five  centimeter1  layer  of  cupric  oxide  in  the 
wire  form  (p.  24)  is  then  introduced  and  an  asbestos 
plug  or  a  short  cupric  oxide  spiral  is  placed  on  top  of  it. 
Owing  to  the  weight  of  the  column  of  cupric  oxide 
the  plug  or  spiral  should  be  supported  from  beneath  by 
the  short  plunger  described  above.  A  ten  centimeter 
space  is  then  left  for  the  boat  in  which  the  substance 
to  be  burned  is  generally  placed  and  the  large  cupric 
oxide  spiral  is  inserted  in  the  tube,  leaving  about  two 
centimeters  between  the  boat  and  the  spiral.  The 
remainder  of  the  tube  is  unoccupied. 

Well-fitting,  one-holed,  red  rubber  stoppers  are  in- 
serted in  each  end  of  the  tube.  The  oxygen  or  air  is 
admitted  at  the  anterior  end  of  the  tube,  i.  e.,  that  con- 
taining the  long  cupric  oxide  spiral,  while  the  prod- 
ucts of  combustion  issue  from  the  other  or  the  exit 
end  of  the  tube.  The  stopper  in  the  anterior  end  is 
generally  furnished  with  a  well-fitting  glass  tube  which 
is  connected  directly  by  means  of  rubber  tubing  with 
the  purifier.  It  is  highly  desirable  to  use  a  straight 
glass  stop-cock  (Fig.  12,  p.  51)  in  place  of  the  glass 
tube.  Where  rubber  and  a  screw  pinch-cock  are  used 
the  cock  often  cuts  the  rubber,  causing  leaks  which 
may  mean  loss  of  carbon  dioxide  and  water.  The 
glass  stop-cock  does  away  with  this  difficulty,  and 

1  A  forty  centimeter  length  of  cupric  oxide  will  suffice  to  burn  most  ma- 
terials, but  a  forty-five  centimeter  length  is  much  safer  to  use. 


BOATS  29 

furthermore  furnishes  a  length  of  tubing  easily  exam- 
ined for  traces  of  condensed  products  of  distillation  or 
sublimed  material  which  has  diffused  backwards  and 
escaped  oxidation  by  the  cupric  oxide  spiral.  Such 
condensation  rarely  occurs,  however,  if  the  combustion 
has  been  properly  conducted.  The  stopper  in  the  exit 
end  of  the  tube  is  connected  directly  with  the  water 
absorbing  tube  of  the  absorption  apparatus.  It  is  im- 
portant, therefore,  to  see  that  the  hole  in  this  cork  is 
of  a  proper  size  to  take  snugly  the  glass  connecting 
tube  of  the  water  absorber. 

The  combustion  tube  thus  prepared  is  placed  in  the 
furnace  and  is  ready  for  the  preliminary  "  burning 
out  "  described  on  page  45. 

BOATS 

The  material  to  be  burned,  if  a  solid  or  a  high  boil- 
ing liquid,  is  almost  invariably  placed  in  a  porcelain, 
copper,  or  platinum  boat  which  is  introduced  into  the 
combustion  tube  by  means  of  a  long  wire  with  a  bend 
on  the  end.  The  boat  should  preferably  have  a  ring 
handle  or  extension  to  facilitate  inserting  and  with- 
drawing it. 

Porcelain. — The  porcelain  boat  should  be  approxi- 
mately seventy-five  millimeters  long  and  not  too  wide 
to  enter  the  combustion  tube  readily.  This  boat  is 
especially  advantageous  for  beginners  as  the  charring 
of  the  material  is  well  seen  against  the  white  back- 
ground. Furthermore,  it  is  seen  at  a  glance  by  the 
absence  of  any  black  color  in  the  boat  when  all  the 
carbonaceous  residue  has  been  oxidized. 


30  ELEMENTARY   ORGANIC   ANALYSIS 

Copper. — Occasionally  substances  are  met  with  that 
are  unusually  refractory  and  require  either  unusual 
heat  or  length  of  time  for  their  complete  oxidation. 
Such  materials  may  often  be  burned  to  advantage  in 
a  copper  boat  which  has  been  heated  till  a  coating  of 
oxide  has  ,been  formed.  While  at  a  high  heat  the  car- 
bonaceous residue  resists  the  action  of  the  oxygen 
alone,  it  is  readily  oxidized  by  the  cupric  oxide  coat- 
ing of  the  copper  with  which  it  is  in  contact  and  the 
copper  thus  reduced  is  again  instantly  oxidized,  there- 
by acting  as  a  carrier  of  oxygen. 

In  burning  substances  with  the  copper  boat  care 
should  be  taken  when  heating  to  prevent  frothing,  and 
the  boat  should  be  covered  with  a  bit  of  sheet  copper 
which  protects  the  tube  from  being  spattered. 

Copper  boats  may  be  obtained  in  the  market.  Owing 
to  their  rapid  disintegration  by  alternate  oxidation 
and  reduction,  it  is  better  to  prepare 
them  as  desired,  using  moderately 
thick  sheet  copper.  A  rectangular 
piece  is  cut  twenty-one  by  sixty-eight 
millimeters  and  by  making  two  folds 
lengthwise  of  the  sheet  a  copper 
trough  of  rectangular  cross-section 
with  a  base  seven  millimeters  wide 
and  sides  seven  millimeters  high  is 
I  I  obtained  (Fig.  6).  A  pair  of  flat- 

Fig.  6.  •  nosed  pliers  are  used  to  bend  up  each 

end  and  a  serviceable  boat  is  easily  made.     A  piece  of 
sheet  copper  sixty-eight  millimeters  long  and  seven 


ABSORBING   AGENTS  31 

millimeters  wide  has  a  strip  five  millimeters  long  bent 
down  at  each  end.  This  cover  when  placed  on  top  of 
the  boat  prevents  spattering  on  the  glass. 

Platinum. — The  expense  of  a  platinum  boat  is  so  great  as 
to  prohibit  its  general  use.  Platinum  absorbs  gases  and 
hence  the  use  of  a  boat  of  this  metal  in  a  current  of  oxygen 
hastens  oxidation  as  does,  in  a  much  greater  degree,  the  use 
of  finely  divided  platinum  in  the  form  of  platinized  asbestos 
or  platinum  sponge. 

ABSORBING  AGENTS 

The  products  of  combustion  issuing  from  the  com- 
bustion tube,  i.  e.,  water  and  carbon  dioxide,  are  re- 
tained in  some  absorbent  and  weighed. 

As  absorbers  of  water  calcium  chloride,  concentrated 
sulphuric  acid,  phosphorus  pentoxide,  and,  at  times, 
solid  potassium  hydroxide  may  be  noted.  Carbon  di- 
oxide maybe  absorbed  by  strong  solutions  of  potassium 
or  sodium  hydroxide,  solid  potassium  hydroxide,  soda- 
lime,1  and  barium  hydroxide. 

Calcium  Chloride  in  the  granulated  form  is  obtained  in  the 
market  and  should  be  sifted  to  remove  all  of  the  finer  powder 
which  is  liable  to  be  mechanically  carried  along  with  the  cur- 
rent of  gas.  It  shtfuld  be  in  granules  of  from  two  to  three 
millimeters  in  diameter.  It  is  well  to  dry  the  chloride  before 
use  by  heating  in  an  evaporating  dish  over  a  Bunsen  flame, 
care  being  taken  not  to  fuse  the  salt.  Fused  calcium  chlo- 
ride, unless  in  small  lumps,  is  not  an  active  dehydrating 
agent,  but  the  coarser,  more  granular,  and  porous  form  may 
be  used  in  larger  pieces.  The  chloride  should  be  chemically 
pure  and 'as  free  from  basic  compounds  as  possible. 

1  Miilder:  Ztschr.  anal.  Chem.,  i,  2  ;  Dennstedt :  "Die  Entwicklung  der 
organischen  Elementaranalyse,"  p.  105  ;  Benedict  and  Tower  :  J.  Am.  Chem. 
Soc.,  21,  389. 


32  ELEMENTARY   ORGANIC   ANALYSIS 

Potassium  hydroxide  in  stick  form  or  in  concentrated  solu- 
tion is  very  generally  used.  The  stick  contains  about  twenty- 
five  per  cent,  of  water.  A  concentrated  solution  is  prepared 
by  dissolving  one  part  of  stick  potassium  hydroxide  in  two 
parts  of  water  and  cooling  the  solution.  Another  method 
consists  of  dissolving  stick  potassium  hydroxide  in  a  small 
amount  of  water  and  diluting  the  cooled  solution  until  its 
specific  gravity  is  I.27.1  If  impurities  in  any  appreciable 
quantity  are  present  in  the  hydroxide  the  solution  should  be 
allowed  to  stand  and  the  supernatant  portion  should  be  de- 
canted off  and  preserved  for  use. 

Soda-lime  when  used  to  absorb  carbon  dioxide  must 
contain  appreciable  quantities  of  moisture;  hence,  the 
dry  fused  soda-lime,  sold  for  use  in  the  determination 
of  nitrogen  by  the  method  of  Will  and  Varrentrapp,  or 
for  drying  gases,  is  not  suited  for  the  absorption  of 
carbon  dioxide. 

A  soda-lime  that  has  given  excellent  satisfaction  is 
quickly  and  easily  prepared  as  follows :  One  kilogram 
of  commercial  caustic  soda,  "  Greenbank  Lye,"  is 
treated  with  750  cc.  of  water  in  an  iron  kettle,  form- 
ing a  strong  solution,  or  more  properly  a  thin  paste. 
While  still  hot  one  kilogram  of  quicklime,  coarsely 
powdered,  is  rapidly  added,  stirring  constantly  with 
an  iron  rod  or  a  piece  of  gas  pipe.  The  lime  is  slaked 
by  the  water  of  the  caustic  soda  solution  and  soon  the 
whole  mass  heats  and  steams.  While  in  this  stage 
it  is  advisable  to  keep  the  mass  stirred  and  the  lumps 
broken.  No  outside  heat  is  necessary  and  as  soon 
as  cool  the  product  is  coarsely  pulverized  and  placed 
in  wide-mouthed  bottles  and  the  corks  sealed  in  with 

1  Auchy  (J.  Am.  Chem.  Soc.,  20,   245)  prefers  a  sp.  gr.  of  1.40. 


ABSORBING   APPARATUS  33 

paraffin  or  wax.  When  cool  it  should  not  be  moist 
enough  to  show  water  as  such,  i.  <?.,  no  particle  should 
glisten  in  strong  light.  If  too  dry,  a  small  quantity 
of  water  can  be  readily  added  after  the  soda-lime  is 
made,  though  the  great  danger  lies  in  adding  too  much 
water,  thereby  making  the  lime  too  pasty  for  the  most 
efficient  absorption.  The  addition  of  dry  soda-lime  to 
a  product  that  is  too  moist  will,  in  many  instances,  ren- 
der it  suitable  for  use.  The  granules  of  soda-lime 
should  not  be  more  than  two  millimeters  in  diameter 
to  secure  the  most  efficient  absorption.  As  the  ma- 
terial is  damp  there  is  no  danger  that  dust  particles 
will  be  carried  along  mechanically  with  the  gas. 

Phosphorus  pentoxide*  is  but  seldom  used  as  an  absorbent 
of  water  in  organic  combustions.  When  used  the  greatest 
efficiency  and  least  possibility  of  clogging  the  tube  is  ob- 
tained by  mixing  the  powder  with  ignited  asbestos.  Asbes- 
tos cord  consisting  of  two  or  three  strands  is  unwound  and 
the  strands  cut  in  pieces  about  one  centimeter  long.  The 
pieces  are  then  strongly  ignited  in  an  iron  dish  and  cooled. 
A  quantity  of  phosphorus  pentoxide  is  placed  in  a  dry  wide- 
mouthed  bottle,  the  pieces  of  asbestos  added,  and  the  bottle 
tightly  corked.  The  bottle  is  well  shaken  and  the  asbestos 
becomes  coated  with  the  powder  which  clings  to  it.  The 
pieces  are  then  quickly  transferred  to  the  tube  in  which  they 
are  to  be  used.  The  hygroscopic  nature  of  this  material  ren- 
ders the  use  of  U-tubes  fitted  with  ground  glass  stoppers 
necessary. 

ABSORBING  APPARATUS 

The  absorbing  system,  though  performing  the  same 
function  as  the  gas  purifier,  must  be  so  constructed  as 

1  I^owe  :  Ztschr.  anal.  Chem.,  n,  403;  Mitscherlich  :  Ibid.,  15,  388  ;  Schmitz  : 
Ibid.,  23,  515. 

3 


34 


ELEMENTARY   ORGANIC    ANALYSIS 


to  conform  to  certain  conditions  in  regard  to  efficiency 
of  absorption,  compactness  of  form,  and  minimum 
weight,  for  it  is  necessary  to  absorb  all  the  products  of 


Fig.  7. 

combustion  in  an  apparatus  not  too  large  or  too  heavy 
to  weigh  on  an  ordinary  analytical  balance.  In  gen- 
eral, the  solid  reagents  are  held  in  the  various  modifi- 
cations of  the  U-tube  and  the  liquid  agents  are  held 
in  the  various  modifications  of  the  u  potash  bulb." 

In  combustions  of  most  organic  substances  more 
water  is  formed  than  is  sufficient  to  saturate  the  gas 
leaving  the  combustion  tube  ;  hence  a  varying  amount 
of  water  condenses  about  the  stopper  in  the  exit  end 
of  the  combustion  tube.  The  water  formed  in  the 
process  of  a  combustion  is  generally  collected  in  a  U- 
tube  having  a  bulb  on  the  arm  nearest  the  combustion 
tube  which  serves  to  collect  the  condensed  moisture 
and  thus  to  prevent  unnecessary  exhaustion  of  the  ab- 
sorbing agent.  As  this  bulb  fills  it  may  be  emptied, 
and  consequently  a  number  of  combustions  may  be 
made  with  the  same  absorbing  tube.  The  Volhard  and 
the  Marchandform  of  U-tube  are  most  commonlv  used. 


ABSORBING   APPARATUS  35 

A  simple  and  efficient  absorbing  system  is  shown 
in  Fig.  7  and  consists  of  three  plain  five-inch  U-tubes 
five-eighths  inch  in  diameter.  The  first  contains  a  small 
glass  vial,  concentrated  sulphuric  acid,  and  glass  wool 
drenched  with  concentrated  sulphuric  acid ;  the  sec- 
ond is  filled  with  soda-lime  prepared  as  described  on 
page  32  ;  the  third  is  one-third  filled  with  soda-lime 
and  the  remaining  volume  with  pumice  stone  drenched 
with  sulphuric  acid. 

In  this  form  of  absorber  the  vial  serves  the  purpose 
of  the  bulb  in  the  other  U-tube.  It  should  be  some- 
what smaller  in  diameter  than  the  U-tube  and  so  sup- 
ported on  a  bit  of  glass  rod  flattened  at  one  end  that 
the  glass  tube  conducts  the  products  of  combustion  into 
the  neck  of  the  vial.  Water  condensing  in  the  tube 
falls  in  drops  to  the  bottom  of  the  vial  and  the  gas 
saturated  with  aqueous  vapor  at  the  temperature  of 
the  apparatus  passes  through  the  U-tube  where  it  is 
dried  and  thence  into  the  carbon  dioxide  absorbers. 

A  plug  of  coarse  glass  wool  is  inserted  in  the  other 
arm  and  extends  from  the  stopper  to  the  point  where 
the  bend  begins.  Enough  commercial  concentrated 
sulphuric  acid  is  slowly  poured  through  the  glass  wool 
to  saturate  it  thoroughly  and  just  seal  the  bend  at  the 
bottom  of  the  U-tube  in  such  a  way  that  the  gas  will 
have  to  bubble  through  it.  The  lower  end  of  the 
glass  wool  will  then  be  about  one  centimeter  above  the 
surface  of  the  liquid,  the  air  space  preventing  too  m  uch 
acid  from  being  carried  up  mechanically  into  the  gl  ass 
wool.  When  this  happens  it  is  not  unusual  for  the 


36  ELEMENTARY    ORGANIC    ANALYSIS 

acid  to  be  carried  over  into  the  carbon  dioxide  ab- 
sorbers. The  air  space  is  otherwise  valuable  as  it  per- 
mits isolation  of  each  bubble  and  consequently  better 
regulation  of  the  rate  of  the  combustion.  If  the  bend 
is  just  sealed  the  minimum  pressure  only  is  necessary 
to  force  the  gas  through  the  tube.  The  greater  por- 
tion of  the  water-vapor  is  retained  by  the  acid  in  the 
bend  of  the  tube  as  the  gas  bubbles  through,  while  the 
last  traces  are  removed  by  the  acid  adhering  to  the 
glass  wool.  Thus  in  one  plain  U-tube  are  incorpo- 
rated three  distinct  drying  operations ;  condensation  of 
excessive  moisture,  removal  of  the  major  part  of  the 
water-vapor,  and  final  absorption  of  remaining  traces 
of  moisture.  The  tubes  are  closed  with  well  fitting 
one-holed  rubber  stoppers  furnished  with  glass  elbows. 
One  elbow  extends  far  enough  below  the  stopper  to  be 
thrust  into  the  neck  of  the  vial.  The  tubes  are  finally 
closed  with  short  bits  of  red  rubber  tubing  fitted  with 
short  glass  plugs.  The  rubber  tubes  are  removed  be- 
fore weighing.  Inasmuch  as  all  the  tubes  in  the  ab- 
sorbing train  can  be  used  for  a  number  of  combustions 
before  refilling,  the  rubber  stoppers  can  be  replaced 
by  corks  which  are  crowded  down  and  cut  off  flush 
with  the  ends  of  the  U-tube.  The  corks  may  then  be 
coated  with  sealing  wax.  This  precaution  is  hardly 
necessary  as  good  rubber  stoppers  give  excellent  re- 
sults. A  tube  prepared  in  this  manner  may  safely  be 
relied  on  to  absorb  about  one  gram  of  water-vapor  ex- 
clusive of  the  water  condensed  in  the  vial.  In  a  series 
of  experiments  on  the  combustion  of  sugar  where  ap- 


ABSORBING   APPARATUS  37 

proximately  0.12  gram  of  water  was  weighed  each 
time,  it  was  found  that  about  three-fourths  of  the  water 
condensed  in  the  vial.  No  direct  estimate  can  be  made 
of  the  length  of  time  such  a  tube  will  last,  owing  to 
the  varying  amounts  of  water  formed  in  different  com- 
bustions, though,  if  the  vial  is  marked  with  a  file 
scratch  at  the  points  indicating  cubic  centimeters,  by 
deducting  the  amount  of  condensed  water  from  the  in- 
crease in  weight  of  the  tube,  a  ready  check  on  the 
amount  of  water  actually  absorbed  is  at  hand. 

The  second  tube  of  the  absorbing  system  is  filled 
with  soda-lime  prepared  as  described  on  page  32,  and 
when  freshly  filled,  will  last  for  from  six  to  fourteen 
combustions,  the  number  depending  on  the  size  of  the 
lumps  of  soda-lime,  the  quantity  in  the  tube,  and  the 
weight  of  carbon  dioxide  absorbed  in  each  combustion. 

The  last  tube  of  the  system  serves  the  dual  purpose 
of  retaining  any  moisture  lost  from  the  soda-lime  tube 
and  any  traces  of  carbon  dioxide  that  may  have  es- 
caped absorption  in  case  the  soda-lime  becomes  ex- 
hausted. With  soda-lime  the  change  in  color  is  a 
very  accurate  indication  of  the  absorption  of  carbon 
dioxide,  and  hence  it  is  only  necessary  to  replace  the 
tube  with  a  fresh  one  before  it  has  been  completely 
whitened.1 

The  increase  in  weight  of  the  last  tube  is  ordinarily 
not  more  than  seven  milligrams  for  each  combustion, 
and  consequently  when  the  increase  is  greater  it  is  an 
additional  sign  that  the  soda-lime  tube  is  nearly  ex- 

1  J.  Am.  Chem.  Soc.,  21,  394. 


38  ELEMENTARY   ORGANIC   ANALYSIS 

hausted.  An  increase  of  more  than  ten  milligrams 
indicates  trie  necessity  of  a  new  tube. 

One  arm  and  the  bend  of  the  last  tube  of  the  system 
are  filled  with  dry  lumps  of  pumice  stone.  Concen- 
trated sulphuric  acid  is  then  allowed  to  trickle  slowly 
down  over  the  pumice  until  it  becomes  thoroughly 
saturated,  but  there  must  not  be  acid  enough  left  in 
the  bottom  of  the  U-tube  to  seal  the  bend.  A  ten 
millimeter  layer  of  glass  wool  is  then  laid  over  the 
pumice  stone  in  the  partially  filled  arm  and  the  re- 
maining space  filled  with  soda-lime.  The  glass  wool 
must  not  come  in  contact  with  the  acid  at  any 
point. 

In  case  sufficient  carbon  dioxide  to  exhaust  the  soda- 
lime  does  not  enter  the  tube,  it  should  last  for  twenty- 
five  or  more  combustions,  since  the  sulphuric  acid 
would  completely  absorb  at  least  one-half  gram  of 
water,  and  if  ten  milligrams  were  retained  from  each 
combustion  the  possible  efficiency  would  be  fifty  com- 
bustions. The  ease  with  which  the  last  tube  is  pre- 
pared, however,  renders  it  more  satisfactory  to  change 
it  after  twenty-five  or  thirty  combustions. 

All  tubes  are  closed  with  two  centimeter  lengths  of 
red  rubber  tubing  fitted  with  bits  of  glass  rod.  The 
edges  of  the  glass  plugs  should  be  fire-polished. 

The  tubes  are  conveniently  placed  in  a  small  paste- 
board box  nineteen  centimeters  long,  thirteen  centi- 
meters high,  and  eight  centimeters  wide,  open  at  the 
top.  Similar  boxes  are  furnished  to  be  used  with  pot- 
ash bulbs. 


ABSORBING   APPARATUS 


39 


Bredt  and  Posth1  have  devised  a  special  form  of  tube  with 
ground-glass  stoppers  for  holding  soda-lime. 

Barium  hydroxide  has  been  used  by  Claesson2andKreusler3 
as  an  absorbent  of  carbon  dioxide.  Its  action  is  similar  to 
that  of  soda-lime. 


Fig.  8. 


Fig.  9. 


Fig.  10. 


Fig.  ii. 


The  most  familiar  and  one  of  the  oldest  absorbing  systems 
consists  of  a  Marchand  (Fig.  9),  or  Volhard4  (Fig.  8)  U-tube 

1  Ann.  Chem.  (I^iebig),  285,  385. 

2  Ber.  d.  chera.  Ges.,  9,  174. 

3  Ztschr.  Chem.  (1866),  292. 

4  Ann.  Chem.  (I^iebig),  176,  339.    The  earlier  forms  of  Marchand  and  Vol- 
hard  tubes  do  not  contain  the  small  extension  tube  in  the  bulb  suggested  by 
Mixter.    This  tube  aids  materially  in  preventing  drops  of  water  entering  the 
arm  of  the  U-tube. 


40  ELEMENTARY   ORGANIC    ANALYSIS 

filled  with  granular  calcium  chloride  followed  by  a  Geissler 
(Fig.  n),  Liebig  (Fig.  10),  or  Mohr  potash  bulb  containing  a 
concentrated  solution  of  potassium  hydroxide. 

The  calcium  chloride  tube  of  this  system  is  designed  to  ab- 
sorb the  water  in  a  number  of  combustions,  consequently, 
after  filling  the  arms  of  the  U-tube  with  the  calcium  chloride 
(page  31)  corks  are  inserted  and  preferably  coated  with  seal- 
ing wax.  At  times  the  arms  to  which  side  tubes  are  attached 
are  closed  by  sealing  off  the  glass  after  filling.  The  tube  is 
provided  with  a  length  of  fine  platinum  or  aluminum  wire  to 
facilitate  suspension  on  the  balance  arm.  This  is  unneces- 
sary if  the  hanger  mentioned  on  page  45  is  used. 

Calcium  chloride  almost  invariably  contains  basic  com- 
pounds which  absorb  not  only  water  but  carbon  dioxide,  and 
which  consequently  introduce  a  serious  error  in  analysis.  It 
is  necessary,  therefore,  to  conduct  a  slow  current  of  dry  car- 
bon dioxide  through  the  tube  after  it  is  all  prepared,  to  neu- 
tralize any  basicity  that  may  exist.  The  tube  is  best  filled 
with  carbon  dioxide,  tightly  closed  with  bits  of  rubber  tubing 
and  glass  plugs  and  left  over  night.  The  unabsorbed  carbon 
dioxide  must  be  carefully  expelled  by  gently  sucking  dry  air 
through  the  tube  for  half  an  hour.  Though  the  efficacy  of 
this  treatment  has  been  doubted,1  probably  the  error  intro- 
duced by  subsequent  absorption  of  carbon  dioxide  is  not  very 
great. 

The  tube  thus  prepared  may  be  used  until  about  one-third 
of  the  calcium  chloride  has  deliquesced.  Lowe2  has  sug- 
gested coating  the  inner  arm  of  the  U-tube  with  tallow  to 
prevent  deliquesced  calcium  chloride  from  adhering '  to  the 
wall  of  the  tube  and  thereby  stopping  the  gas.  In  case  this 
method  is  used  obviously  the  limbs  of  the  tube  cannot  be 
sealed  off. 

The  ' '  potash  bulb  ' '  of  Liebig  is  the  earliest  form  of  appa- 
ratus for  holding  concentrated  potassium  hydroxide  to  absorb 

1  Winkler  :  Ztschr.  anal.  Chem.,  ai,  545  (1882). 

2  Ibid.,  11,403  (1872). 


ABSORBING   APPARATUS  41 

carbon  dioxide  in  organic  combustions.  This  bulb  has  been 
largely  superceded  by  the  more  elaborate  and  fragile  bulbs  of 
Geissler  and  Mohr  which  have  the  advantage  that  they  can 
be  placed  on  the  pan  of  the  balance  and  need  no  support 
while  the  Liebig  bulb  must  of  necessity  be  suspended.  The 
potassium  hydroxide  solution  is  introduced  into  a  Liebig 
bulb  by  applying  gentle  suction  with  the  mouth  through  a 
rubber  tube  attached  at  one  end  of  the  bulb.  The  other  end 
is  dipped  in  the  potassium  hydroxide  solution  and  sufficient 
liquid  is  drawn  into  the  apparatus  nearly  to  fill  the  three 
bulbs  on  the  lower  arm.  The  end  which  was  dipped  into  the 
concentrated  potassium  hydroxide  is  then  carefully  cleaned 
inside  and  out,  introducing  long  pointed  rolls  of  filter-paper 
to  absorb  the  excess  of  potassium  hydroxide  .adhering  to  the 
inside  of  the  glass  tube.  When  the  bulb  is  filled  the  ends 
are  closed  with  bits  of  rubber  tube  and  glass  plugs.  Such  a 
bulb  should  be  refilled  after  two  combustions.  A  prolong 
consisting  of  a  small  straight  ' ' calcium  chloride  ' '  tube,  filled 
with  small  lumps  of  potassium  hydroxide,  is  attached  to  the 
exit  end  of  the  Liebig  bulb  to  retain  any  moisture  lost  from 
the  concentrated  potassium  hydroxide  as  well  as  any  traces 
of  carbon  dioxide  escaping  absorption. 

The  Geissler  bulb  requires  a  little  more  care  in  filling  with 
concentrated  potassium  hydroxide.  The  prolong,  if  attached, 
is  removed  and  a  rubber  tube  is  connected  with  the  arm  of 
the  tube  on  which  the  smaller  bulb  is  blown.  When  the  pro- 
long is  connected  with  a  ground-glass  joint  the  rubber  tube 
is  slipped  over  the  ground  end  on  the  limb.  The  other  end 
is  then  dipped  in  concentrated  potassium  hydroxide  solution 
(p.  32)  and  sufficient  liquid  is  drawn  into  the  apparatus  to 
fill  each  of  the  three  lower  chambers  a  little  more  than  half 
full.  The  filling  is  easily  accomplished  by  inclining  the  bulb 
and  applying  gentle  suction.  The  greatest  care  must  be 
taken  to  avoid  sucking  concentrated  lye  into  the  mouth.  An 
empty  wash-bottle  may  be  inserted  between  the  mouth  and 


42  ELEMENTARY   ORGANIC   ANALYSIS 

the  bulb  or  a  very  gentle  suction  with  the  filter-pump  may 
be  used.  The  suction  should  first  be  started  and  then  the 
tube  connected  with  the  bulb.  After  filling  the  bulb  the 
tube  that  was  dipped  in  the  liquid  should  be  carefully  cleaned 
as  described  above  and  the  apparatus  plugged  ready  for 
weighing.  The  solution  should  be  renewed  after  two  com- 
bustions. 

The  prolong  contains  either  stick  potassium  hydroxide, 
fused  soda-lime,  or  granulated  calcium  chloride,  and  serves 
to  retain  the  water  vaporized  from  the  potassium  hydroxide 
solution  as  well  as  any  traces  of  carbon  dioxide.  While  solid 
potassium  hydroxide  has  been  used  for  many  years  the  use 
of  calcium  chloride  is  more  logical  as  the  gas  entering  the 
bulb  is  dried  over  calcium  chloride  and  hence  should  be  dried 
over  this  reagent  before  leaving.  If  the  prolong  is  filled  with 
calcium  chloride  it  is  unnecessary  to  conduct  carbon  dioxide 
through  it  before  using. 

Of  the  other  modifications  of  the  potash  bulb  those  of  Mit- 
scherlich,1  Winkler,2  De  Koninck,3  Kyll,4  Delisle,*  Gomberg,6 
Bender  and  Hobein,  and  Bowen  may  be  mentioned. 

WIPING  AND  WEIGHING  THE  APPARATUS 

The  greatest  difficulty  is  experienced  in  securing 
the  correct  weight  of  the  absorption  apparatus  before 
and  after  a  combustion  on  account  of  the  great  inequality 
in  the  amount  of  moisture  and  gases  condensed  on  the 
surface  of  the  glass.  This  inequality  is  often  of  suffi- 
cient magnitude  to  render  an  analysis  useless.  At- 
tempts are  made,  therefore,  to  have  the  apparatus  at 

1  Ztschr.  anal.  Chem.,  15,  389. 

2  Ibid.,  21,  545. 

3  Ber.  d.chem.  Ges.,  3,  287. 

4  Chem.  Ztg.,  18,  1006. 

5  Ber.  d.  chem.  Ges.,  24,  271. 

6  J.  Am.  Chem.  Soc.,  18,  941. 


WIPING   AND   WEIGHING   THE   APPARATUS.  43 

the  same  temperature  and  with  the  same  surface  con- 
densation after  the  combustion  as  before.  To  this  end 
it  is  usually  recommended  that  the  apparatus  should 
be  lightly  wiped  with  a  clean  soft  cloth  and  then 
allowed  to  stand  a  half  hour  beside  the  balance  before 
being  weighed.  This  operation  is  fairly  effective  in 
good  weather  and  doubtless  gives  a  close  approxima- 
tion to  correct  results,  but  the  differences  in  the  amount 
of  condensation  on  days  in  which  the  atmospheric  con- 
ditions are  not  the  same,  are  very  considerable.  When 
the  conditions  to  which  the  glass  is  exposed  in  the 
course  of  the  combustion  are  considered,  i.  e.,  the 
wiping,  the  handling  with  moist  fingers,  the  sojourn 
of  at  least  an  hour  in  close  proximity  to  a  combustion 
furnace,  together  with  the  considerable  internal  heat 
from  the  absorption  of  the  carbon  dioxide  by  the  re- 
agent, the  assumption  that  the  surface  condition  re- 
mains the  same  after  as  before,  even  with  all  precau- 
tions, is  rather  broad. 

In  an  operation  of  this  kind  obviously  the  simpler 
the  form  of  absorber  and  the  smaller  the  surface,  the 
more  readily  can  the  apparatus  be  brought  into  con- 
dition for  weighing.  With  the  absorber  first  described 
(p.  34)  it  is  possible  to  have  constant  surface  conditions 
before  and  after  analysis  and  thus  be  independent  of 
the  weather.  By  carefully  wiping  the  U-tubes  with 
clean,  dry  cheesecloth,  it  is  possible  to  clean  the  ap- 
paratus till  there  is  no  longer  any  loss  in  weight.  This 
point  is  taken  as  the  standard  condition  and  the  tubes 
are  so  wiped  before  and  after  each  combiistion. 


44  ELEMENTARY   ORGANIC    ANALYSIS 

The  rubber  plugs  are  removed  from  the  first  U-tube 
and  the  tube  wiped  thoroughly  with  a  piece  of  clean, 
dry  cheesecloth  in  each  hand  in  such  a  way  that  the 
glass  does  not  come  in  contact  with  the  fingers.  It  is 
necessary  to  give  the  tube  a  hard,  thorough  rubbing. 
The  tube  is  then  placed  on  the  balance,  brought  to 
equilibrium,  then  removed  and  thoroughly  wiped  again 
and  weighed.  It  will  probably  have  lost  somewhat 
in  weight.  The  operation  is  repeated  until  the  weight 
remains  constant.  After  a  little  experience  it  is  sel- 
dom necessary  to  wipe  the  tube  more  than  three  times. 
The  tube  is  then  plugged  and  the  weight  recorded. 
At  the  end  of  the  combustion  the  operation  is  repeated 
and  the  condensation  on  the  surface  of  the  glass  there- 
by eliminated.  The  importance  of  using  clean,  dry 
cheesecloth  cannot  be  too  much  emphasized.  A  tube 
so  cleaned  rapidly  increases  in  weight  owing  to  the 
condensation  on  its  surface,  but  the  increase  is  not  too 
rapid  to  prevent  making  an  accurate  record  of  the 
weight. 

This  method  would  be  impossible  if  applied  to  a 
Liebig  or  a  Geissler  potash  bulb.  The  fragile  nature 
of  such  bulbs  would  not  permit  of  careful,  thorough 
wiping  of  their  excessively  large  surface,  and  conse- 
quently the  procedure  must  be  that  usually  recom- 
mended, outlined  on  page  43.  The  use  of  a  counter- 
poise when  weighing  potash  bulbs  has  been  recom- 
mended.1 The  counterpoise  should  consist  of  a  simi- 

1  Regnault  :  Handworterbuch,  Suppl.,  189  ;  Blair  cited  by  Dudley  :  J.  Am. 
Chetn.  Soc.,  19,  96. 


BURNING   OUT   THE    COMBUSTION   TUBE  45 

lar  bulb  which,  however,  is  not  filled  with  the  reagent. 
Each  tube  and  piece  of  apparatus  may  be  provided 
with  a  loop  of  fine  platinum  or  aluminum  wire  though 
it  is  much  easier  to  use  a  hook  made  from  a  piece  of 
stout  copper  wire  so  bent  as  to  hang  on  the  arm  of  the 
balance  and  furnish  suspension  for  the  U-tubes. 

WEIGHT  OF  MATERIAL  USED 

The  amount  of  material  used  is  generally  dependent 
on  the  amount  of  carbon  it  contains.  From  two-  to 
three-tenths  of  a  gram  is  ordinarily  used.  Where  the 
approximate  composition  of  the  substance  is  known 
it  is  advisable  to  use  an  amount  that  will  yield,  when 
burned,  from  three-  to  four-tenths  of  a  gram  of  carbon 
dioxide.  In  case  of  compounds  of  higher  carbon  con- 
tent the  carbon  dioxide  formed  may  rise  to  five-tenths 
of  a  gram,  but  should  not  ordinarily  exceed  that 
amount. 

In  burning  compounds  mixed  with  sugar  or  ben- 
zoic  acid,  as  described  on  page  60,  the  carbon  dioxide 
from  the  admixture  must  be  considered  in  calculating 
the  total  amount  that  must  be  absorbed. 

BURNING  OUT  THE  COMBUSTION  TUBE 

When  a  new  tube  is  filled  there  are  always  particles 
of  dust  and  a  considerable  quantity  of  moisture  ad- 
hering to  the  tube  and  the  materials,  which  must  of 
necessity  be  removed  before  making  an  analysis.  This 
is  most  readily  accomplished  by  heating  the  tube  in 
the  furnace  and  conducting  a  current  of  oxygen  through 


46  ELEMENTARY   ORGANIC   ANALYSIS 

it.  The  moisture  is  all  expelled  by  the  heat  and  the 
current  of  gas,  while  all  organic  matter  is  completely 
oxidized  to  water  and  carbon  dioxide  which  are  finally 
expelled  by  dry  oxygen.  An  old  tube  which  has  stood 
some  time  since  being  used  should  likewise  be  burned 
out.  The  tube  filled  as  described  on  page  27  is  placed 
in  the  combustion  furnace,  a  slow  current  of  pure,  dry 
oxygen  is  conducted  through  it,  and  the  heat  gradu- 
ally raised  until  the  whole  tube  is  heated  to  a  low  red. 
No  operation  connected  with  the  making  of  an  or- 
ganic combustion  is  attended  with  as  many  accidents 
as  is  the  heating  of  the  tube.  The  utmost  care  must 
be  exercised  to  secure  an  even  heating  which  is  grad- 
ually increased  to  the  desired  point.  If  Jena  glass  is 
used,  and  proper  care  is  exercised  in  protecting  the 
tube  by  layers  of  asbestos  in  the  iron  or  tile  trough, 
and  in  applying  the  heat  judiciously,  no  difficulty 
should  be  experienced.  At  the  start  the  burners 
should  all  be  turned  on  full  force,  the  air  holes  closed 
and  the  combustion  tube  either  removed  from  the  fur- 
nace or  supported  on  the  tiles  used  to  cover  the  tube 
during  combustion  in  such  a  manner  that  it  is  not 
heated.  The  gas  is  then  turned  on  full  at  the  main 
cock,  allowed  to  rush  through  the  burners  a  moment, 
then  it  is  nearly  shut  off,  and  the  burners  are  lighted 
from  beneath.  The  flame  will  run  along  till  all  the 
burners  are  lighted  and  burning  with  a  smoky,  lumi- 
nous flame.  The  gas  is  then  turned  down  till  the 
flames  are  not  more  than  seven  to  ten  millimeters  high 
and  the  tube  is  placed  in  the  trough.  In  five  minutes 


BURNING   OUT   THE    COMBUSTION   TUBE  47 

the  tiles  are  closed  over  the  tube  and  at  the  end  of  five 
more  minutes  the  gas  is  turned  on  more  and  the  flames 
made  non-luminous  by  opening  the  air  holes  at  the 
base  of  the  burners.  The  gas  should  be  turned  on  at 
each  burner  till  the  flame  is  non-luminous  without 
striking  back.  After  this  stage  the  heat  may  be  in- 
creased with  greater  rapidity  until  the  desired  temper- 
ature is  attained. 

The  temperature  to  which  the  tube  should  be  heated 
is  variously  given  from  just  below  red  heat  to  a  cherry- 
red  which  limits  would  extend  from  500°  to  1000°  C. 

With  Jena  glass  tubing  the  heat  may  be  safely  car- 
ried to  a  strong  cherry-red  without  danger  of  having 
the  tube  blow  out  or  become  very  much  distorted.  By 
properly  supporting  the  absorbing  system  and  the  glass 
stop-cock  in  the  entrance  end  of  the  tube  so  as  to  bring 
no  great  leverage  on  the  tube  no  appreciable  bending 
will  occur. 

With  Bohemian  glass  the  heat  must  be  much  more 
carefully  regulated  and  not  carried  beyond  a  low  red 
heat.  In  either  case  unnecessary  heating  is  to  be 
avoided  as  wasteful  of  gas  and  deleterious  to  the  tube. 
A  low  red  heat  will  serve  to  burn  most  all  substances. 

A  new  tube  should  be  heated  for  at  least  half  an 
hour  with  a  continual  current  of  oxygen  passing 
through  it.  At  the  end  of  that  time  a  short  straight 
calcium  chloride  tube  filled  with  the  granular  chloride, 
which,  however,  needs  no  preliminary  treatment  with 
carbon  dioxide,  is  inserted  in  the  cork  in  the  exit  end 
of  the  tube.  The  small  cork  in  the  open  end  of  the 


48  ELEMENTARY   ORGANIC    ANALYSIS 

calcium  chloride  tube  is  fitted  with  a  small  glass  tube 
which  is  purposely  drawn  down  to  a  point,  thereby 
allowing  expansion  and  contraction  of  the  gas  inside 
the  tube  while  permitting  the  minimum  diffusion  of 
air  through  the  tip  when  the  combustion  tube  is  cold. 
Under  these  conditions,  after  cooling,  the  tube  may  be 
allowed  to  stand  some  time  and  may  be  used  for  a  com- 
bustion without  previous  burning  out.  The  calcium 
chloride  tube  is  effective  when  the  intervals  between 
use  are  short;  i.  e.,  not  over  one  or  two  days,  otherwise 
the  tube  should  be  burned  out. 

In  cooling  a  combustion  tube  care  must  be  taken  to 
avoid  too  sudden  changes  in  temperature.  With*  Bo- 
hemian glass  tubing  the  diminution  of  heat  must  be 
made  as  gradual  as  possible.  The  flames  are  gradu- 
ally lowered  at  intervals  of  three  or  four  minutes  until 
it  is  necessary  to  cut  off  the  air  supply  to  keep  them 
from  striking  back.  The  luminous  flames  about  one 
centimeter  high  are  retained  for  from  five  to  ten  min- 
utes longer  and  the  gas  is  finally  cut  off.  The  tiles 
covering  the  tube  must  remain  closed  till  the  tube 
cools. 

With  Jena  glass  tubing  the  cooling  process  is  much 
more  rapid.  As  a  general  rule  the  entire  supply  of 
gas  may  be  cut  off  at  once,  leaving  the  tiles  closed, 
without  damage  to  the  tube.  It  is  a  little  safer,  how- 
ever, first  to  turn  the  gas  off  one-half  for  five  minutes 
and  then  cut  off  completely.  In  any  case  the  tiles 
should  remain  over  the  tube. 

When  a  combustion  is  to  be  made,  immediately  after 


GENERAL   PROCESS   OF  THE   COMBUSTION  49 

burning  out  the  tube,  or  when  successive  combustions 
are  to  be  made,  it  is  unnecessary  to  cool  the  whole 
combustion  tube.  Usually  the  anterior  portion,  i.  e., 
that  containing  the  cupric  oxide  spiral  and  the  first 
fifteen  centimeters  of  the  layer  of  cupric  oxide,  alone  is 
cooled  until  the  hand  can  be  comfortably  placed  upon 
it  while  the  gas  flames  under  the  remainder  are  low- 
ered till  the  tube  is  just  below  a  dull  red  heat.  This 
is  accomplished  by  lowering  the  flames  under  the  an- 
terior portion  gradually  at  intervals  of  three  or  four 
minutes  and  finally  cutting  off  the  gas  entirely.  The 
tiles  are  then  thrown  back  and  the  tube  allowed  to 
cool. 

With  Jena  glass  tubes  the  gas  under  the  anterior 
portion  is  completely  cut  off  at  once1  and  the  tiles 
thrown  back  after  five  minutes.  As  soon  as  the  hand 
can  be  held  on  the  tube  it  is  sufficiently  cool  and  ready 
for  the  introduction  of  the  substance  to  be  burned. 
Obviously  the  degree  to  which  the  tube  should  be 
cooled  depends  upon  the  volatility  of  the  substance  to 
be  analyzed.  (See  page  73.) 

GENERAL  PROCESS  OF  THE  COMBUSTION 

The  combustion  tube  is  first  heated  in  a  gentle  cur- 
rent of  air  or  oxygen,  as  described  on  page  45,  to  re- 
move moisture,  and  then  the  anterior  portion  inclu- 
ding about  fifteen  centimeters  of  the  cupric  oxide  layer 
cooled  down  till  the  hand  can  be  held  on  it.  The  cur- 

1  When  cutting  off  any  considerable  quantity  of  gas  it  is  important  to  see 
that  the  remaining  flames  do  not  burn  unnecessarily  high  by  reason  of  the 
greater  supply  of  gas. 

4 


50  ELEMENTARY   ORGANIC   ANALYSIS 

rent  of  air  or  oxygen  is  then  cut  off  by  means  of  the 
glass  stop-cock  or  screw  pinch-cock,  and  after  remov- 
ing the  small  calcium  chloride  tube  the  water-absorb- 
ing tube  is  inserted  in  the  cork  in  the  exit  end  of  the 
combustion  tube.  The  U-tube  is  suspended  on  a  wire 
hook  resting  on  a  long  glass  rod  horizontally  clamped 
(Fig.  12).  The  carbon  dioxide  absorber  is  then  at- 
tached by  means  of  the  short  bit  of  red  rubber  tubing 
used  as  a  plug.  To  insure  good  connections  the  ends  of 
the  glass  tubes  should  touch  inside  the  rubber  tube. 
This  is  especially  necessary  when  using  potash  bulbs 
owing  to  the  considerable  pressure  required  to  force 
the  gas  through  the  liquid.  By  using  a  small  aspira- 
tor1 the  pressure  may  be  neutralized.  In  case  the  as- 
pirator is  used  it  is  advisable  to  insert  an  unweighed 
calcium  chloride  tube  between  the  aspirator  and  the 
absorbing  system  to  prevent  back  diffusion  of  mois- 
ture. Such  a  guard  tube  may  also  be  used  after  any 
system  of  absorbers,  though  if  the  combustion  is  prop- 
erly conducted  there  is  no  necessity  of  having  back 
suction. 

After  connecting  the  absorbing  system  to  the  tube 
the  cork  is  removed,  the  long  cupric  oxide  spiral  with- 
drawn upon  an  iron,  porcelain  or  previously  ignited 
asbestos  plate  by  means  of  a  long  wire  with  a  hook  on 
the  end,  and  the  boat  containing  the  weighed  sub- 
stance is  carefully  thrust  into  the  tube.  The  boat  is 
pushed  along  the  tube  until  within  one  centimeter  of 
the  asbestos  plug.  Since  the  tube  becomes  discolored 

1  Glaser:  Ann.  Chem.  (L,iebig),  Suppl.,  7,  213. 


GENERAL   PROCESS   OF  THE   COMBUSTION 


and  opaque  with  age  it  is  advis- 
able to  insert  the  boat  by  pla- 
cing the  hook  of  the  wire  in  the 
ring  at  the  rear  of  the  boat, 
pushing  it  into  the  tube  until 
it  touches  the  asbestos,  with- 
drawing it  one  centimeter  and 
raising  the  hook  out  of  the  ring. 

The  cupric  oxide  coil  is 
thrust  into  the  tube  to  within 
two  or  three  centimeters  of  the 
boat  and  the  cork  inserted. 
This  operation  may  be  quite 
deliberate  if  the  substance  is 
not  volatile,  but  in  the  case  of 
volatile  compounds  the  intro- 
duction should  be  as  rapid  as 
possible. 

The  cupric  oxide  coil  is  first 
heated  to  prevent  back  distilla- 
tion of  volatile  organic  material, 
and  when  the  coil  is  hot  enough 
to  oxidize  such  material  the 
boat  is  heated.  As  a  rule  the 
three  burners  directly  under 
the  coil  are  lighted,  turned  on 
full,  and  the  tiles  above  closed. 
The  tiles  covering  the  boat 
should  be  opened  for  five  centi- 
meters on  each  side  of  the  boat 


52  ELEMENTARY   ORGANIC   ANALYSIS 

to  prevent  premature  heating  of  the  boat.  If  an  iron 
trough  is  used  the  heat  will  be  conducted  along  the 
trough  and  heat  the  tube,  though  the  layers  of  asbestos 
will  serve  to  protect  the  tube  somewhat. 

The  greatest  difficulty  in  the  whole  operation  of 
making  an  organic  combustion  is  heating  the  substance 
properly. 

As  soon  as  the  cupric  oxide  coil  is  at  a  low  red  heat 
the  tiles  over  the  boat  are  closed  and  the  boat  is  grad- 
ually heated.  It  should  be  borne  in  mind  that  the 
heat  is  conducted  along  the  iron  trough  and  conse- 
quently those  burners  nearest  the  boat  should  be  lighted 
only  after  some  time.  The  whole  operation  is  easily 
regulated  by  the  rate  of  bubbling  in  the  sulphuric  acid 
U-tube  of  the  absorbing  system  (Fig.  7),  or  in  the 
potash  bulb  in  case  potassium  hydroxide  is  used  to  ab- 
sorb carbon  dioxide.  In  case  a  "5/8  inch"  U-tube 
containing  sulphuric  acid  (Fig.  7)  is  used  the  bubbling 
should  not  be  faster  than  one  bubble  per  second.  With 
potash  bulbs  which  contain  tubes  of  much  smaller 
caliber  two  bubbles  per  second  are  permissible.  This 
ratio,  i  :  2,  by  no  means  expresses  the  relation  of  the 
volumes  of  gas  passing  the  tubes  as  the  rate  is  much 
greater  in  the  U-tube.  Potassium  hydroxide  is  not  so 
good  an  absorbent  of  carbon  dioxide  as  soda-lime,  hence 
the  current  of  gas  must  be  much  slower. 

During  the  preliminary  heating  of  the  cupric  oxide 
coil  the  air  in  the  tube  will  expand  and  bubble  through 
the  liquid.  As  the  temperature  approaches  redness 
the  bubbling  will  be  much  slower.  Heating  the  boat 


GENERAL   PROCESS   OF   THE    COMBUSTION  53 

will  again  cause  an  expansion  of  air  in  that  portion  of 
the  tube  and  consequent  increase  in  the  rate  of  bub- 
bling. As  soon  as  the  material  begins  to  volatilize 
and  either  through  sublimation  or  dry  distillation  to 
come  in  contact  with  the  hot  cupric  oxide,  it  is  oxi- 
dized to  form  water  and  carbon  dioxide  which  increase 
the  volume  of  gas  inside  the  tube  and  again  increase 
the  bubbling.  As  soon  as  the  bubbling  indicates  an 
increase  in  the  evolution  of  gas  the  burners  nearest  the 
boat  are  turned  off  and,  if  necessary,  the  tiles  thrown 
back. 

To  be  on  the  safe  side,  it  is  advisable  to  heat  very 
slowly  and  at  each  indication  of  rapidly  increasing 
rate  of  bubbling,  to  cool  the  tube  considerably  by  cut- 
ting off  the  gas  burners  nearest  the  boat  and  opening 
the  tiles.  The  cooling  should  be  rapid  and  one  should 
err  on  the  side  of  cooling  too  much  as  by  subsequent 
heating  the  combustion  is  immediately  continued.  As 
soon  as  bubbling  becomes  very  slow  heat  may  again 
be  cautiously  applied.  By  alternately  warming  and 
cooling  (taking  care  to  anticipate  any  great  increase 
in  the  rapidity  of  bubbling)  the  combustion  may  be 
so  regulated  that  oxidation  is  complete  and  no  carbon 
dioxide  or  water  escapes  absorption  in  the  absorbing 
system. 

Volatile  substances  gradually  sublime  and  some  time 
is  required  to  burn  all  the  material.  Some  substances, 
such  as  sugar,  melt  and  char  rapidly,  giving  off  a  great 
deal  of  water-vapor  and  gases,  leaving  a  charred  resi- 
due. The  manipulation  varies  with  the  substance, 


54  ELEMENTARY   ORGANIC    ANALYSIS 

but  only  in  the  case  of  very  volatile  bodies  or  sub- 
stances more  or  less  explosive  is  any  great  difficulty 
experienced  in  regulating,  by  proper  manipulation  of 
flames  and  tiles,  the  rate  of  combustion. 

When  the  "bubbling  has  nearly  ceased  a  very  gentle 
current  of  oxygen  is  admitted,  the  rate  being  observed 
by  the  bubbling  in  the  purifying  apparatus  rather  than 
in  the  absorbing  system.  As  a  large  quantity  of  cupric 
oxide  has  been  reduced  the  oxygen  will  all  be  absorbed 
at  first  in  reoxidizing  the  cupric  oxide  spiral.  If  the 
gas  current  is  very  rapid  as  soon  as  the  free  oxygen 
reaches  the  charred  residue  in  the  boat,  carbon  dioxide 
will  be  formed  and  a  great  rush  of  gas  will  force  its  way 
through  the  absorbers  with  the  consequent  liability  of 
escape  of  unoxidized  material.  A  regular  bubbling 
of  the  oxygen  in  the  purifier  is  the  only  safe  guide. 
The  charred  mass  will  glow  and  burn  brightly  in  the 
oxygen.  This  will  serve  to  indicate  the  progress  of 
combustion  if  the  tube  is  too  discolored  to  be  trans- 
parent. As  soon  as  the  contents  of  the  boat  have  been 
entirely  oxidized  the  anterior  portion  of  the  tube  may 
be  cooled  ready  for  a  second  combustion.  The  current 
of  oxygen  is  continued  till  a  good  test  for  oxygen  is 
obtained  at  the  open  end  of  the  last  tube  in  the  ab- 
sorbing system.  Splinters  made  of  cigar-box  wood 
retain  their  sparks  a  long  time  and  are  especially  to 
be  recommended  for  this  purpose. 

As  soon  as  the  oxygen  is  admitted  the  moisture  con- 
densed in  the  entrance  end  of  the  combustion  tube 
should  be  driven  off  by  playing  a  Bunsen  flame  over 


GENERAL   PROCESS   OF   THE   COMBUSTION  55 

the  surface  of  the  glass,  exercising  care  not  to  over- 
heat the  rubber  stopper.  As  soon  as  the  cooling  pro- 
cess is  begun  the  moisture  condensed  in  the  exit  end 
of  the  tube  should  be  in  a  similar  manner  driven  over 
into  the  water-absorbing  tube. 

The  combustion  requires  in  general  about  one  hour 
from  the  introduction  of  the  boat  to  the  removal  of 
the  tubes.  Five  to  ten  minutes  are  required  to  heat 
the  cupric  oxide  spiral ;  twenty  minutes  to  heat  the 
material  in  the  boat  till  no  more  gas  bubbles  through 
the  liquid  in  the  absorbers ;  fifteen  minutes  till  the 
tube  is  ready  to  cool  ;  and  fifteen  minutes  to  cool  the 
tube  and  get  ready  for  the  next  combustion.  During 
the  last  half  hour  oxygen  is  conducted  through  the 
combustion  tube  and  consequently  all  unoxidized  or- 
ganic material,  as  well  as  the  copper,  is  thoroughly 
oxidized  and  the  products  of  combustion  are  swept 
out  of  the  combustion  tube  leaving  the  tube  ready  for 
another  combustion.  About  one  and  a  half  liters  of 
oxygen  are  required  for  the  combustion.  With  ma- 
terials which  volatilize  unchanged  and  leave  no  car- 
bonaceous residue,  it  is  necessary  to  heat  a  longer  time 
before  admitting  the  oxygen,  but  on  the  other  hand, 
less  time  is  required  to  complete  the  oxidation  with 
oxygen.  After  oxygen  is  admitted  the  combustion 
practically  runs  itself,  save  for  the  time  required  to  cut 
off  the  gas  and  cool  the  anterior  portion  of  the  com- 
bustion tube. 

When  succeeding  combustions  are  to  be  made,  the 
absorbers  may  be  weighed,  filled  with  oxygen  be- 


56  ELEMENTARY   ORGANIC   ANALYSIS 

fore  and  after  use.  The  openings  through  the  glass 
elbows  or  connecting  arms  are  so  small  as  to  prevent 
any  considerable  diffusion  during  the  short  time  they 
are  open  for  weighing.  The  weight  of  the  set  removed 
would,  of  course,  be  taken  as  the  original  weight  of 
the  set  for  a  new  combustion. 

It  is  better,  however,  unless  one  is  unusually  skilled 
in  combustion  work  to  replace  the  oxygen  by  dry  air. 
A  U-tube  similar  to  the  last  tube  in  the  absorbing 
system  (Fig.  7),  is  used  to  free  the  air  from  moisture 
and  carbon  dioxide.  As  usually  conducted  the  limb 
of  the  purifying  tube  containing  the  greater  quantity 
of  pumice  stone  is  connected  by  a  short  rubber  con- 
nector to  the  entrance  end  of  a  water-absorbing  tube, 
i.  e.,  the  limb  of  the  U-tube  (Fig.  7)  containing  the 
glass  vial.  A  twenty  centimeter  length  of  small  rub- 
ber tubing,  having  an  eight  centimeter  length  of  small 
glass  tube  inserted  in  one  end,  is  connected  to  the 
exit  limb  of  the  water  absorber.  The  short  glass  tube 
is  placed  in  the  mouth  and  air  is  drawn  through  the 
tube  for  a  few  moments.  The  air  taken  in  at  one  long 
breath  is  sufficient  to  remove  all  oxygen.  Obviously 
the  air  current  must  not  be  rapid  enough  to  cause  the 
acid  to  bubble  up  into  the  glass  wool  and  run  out  of 
the  tube. 

The  carbon  dioxide  absorber  is  likewise  connected 
with  the  purifying  U-tube  and  two  long  breaths1  of 
air  drawn  slowly  through  the  system.  In  case  it  con- 

1  Dudley  and  Pease  (J.  Am.  Cliem.  Soc.,  15,  540)  find,  when  using  a  small 
potash  bulb,  that  eight  hundred  cc.  of  air  should  be  aspirated  to  remove  al 
oxygen.  Much  less  would  be  required  when  a  soda-lime  tube  is  used. 


GENERAL   PROCESS   OF   THE   COMBUSTION  57 

sists  of  two  pieces  (Fig.  7),  both  should  be  connected 
as  used  in  order  that  the  moisture  lost  from  the  soda- 
lime  (or  potassium  hydroxide)  may  be  retained  in  the 
second  tube  or  prolong. 

The  same  care  must  be  exercised  in  preparing  the 
purifying  tube  as  is  used  in  preparing  the  absorbing 
tube  described  on  page  38.  Sufficient  glass  wool  must 
be  between  the  soda-lime  and  the  acid.  A  great  ex- 
cess of  acid  is  to  be  avoided.  The  purifying  tube 
should  last  for  twenty-five  or  thirty  combustions.  It 
is  then  well  to  refill  it.  The  rate  at  which  the  air  is 
being  drawn  through  may  be  readily  determined  by 
connecting  an  instant  with  a  gas  washing-bottle  or 
the  mouth  tube  of  an  ordinary  wash-bottle.  Air  will 
bubble  through  the  water  and  after  a  few  trials  the 
rate  is  easily  controlled.  With  a  little  experience  no 
indicator  is  necessary.  If  too  much  air  is  drawn 
through  the  wash-bottle  the  soda-lime  will  rapidly  be- 
come pasty  and  inefficient. 

Burning  in  a  closed  tube,  i.  e.,  with  no  current  of 
oxygen,  is  liable,  under  certain  circumstances,  to  cause 
a  slight  back  suction.  At  times  this  is  caused  by  the 
scaling  off  of  cupric  oxide  from  the  large  spiral,  ex- 
posing fresh  surfaces  of  unoxidized  copper  which  im- 
mediately absorb  oxygen,  resulting  in  a  contraction 
in  volume.  In  this  case  if  the  spiral  is  hot  enough  to 
absorb  oxygen  the  preliminary  steps  in  heating  the 
material  in  the  boat  may  be  taken  and  the  expansion 
of  the  air  in  the  tube  will  soon  stop  the  back  suction. 
If  this  is  not  immediately  effective  a  very  slow  stream 


58  ELEMENTARY   ORGANIC    ANALYSIS 

of  oxygen  may  be  admitted,  just  enough  to  prevent 
back  suction,  but  not  enough  to  cause  the  gas  to  bub- 
ble out  of  the  tube.  As  soon  as  the  tendency  to  back 
suction  is  overcorne  the  oxygen  is  cut  off.  Sufficient 
oxygen  may  be  introduced  into  the  purifying  appara- 
tus to  cause  a  slight  pressure.  By  slowly  opening  the 
stop-cock  one  or  more  bubbles  of  oxygen  may  be  ad- 
mitted to  the  combustion  tube  to  prevent  back  suction. 

COMBUSTION  OF  NITROGENOUS  SUBSTANCES 

Certain  compounds  containing  nitrogen  when  de- 
composed yield  varying  quantities  of  nitric  oxide 
which  reacts  with  oxygen  or  air  and  water,  forming 
many  of  the  series  of  oxides  and  oxy-acids  of  nitrogen 
which  go  under  the  general  name  of  the  oxides  of 
nitrogen.  Chief  among  these  is  nitrogen  peroxide 
which  often  appears  in  the  combustion  tube  as  red 
fumes.  While  neither  nitrogen  nor  nitric  oxide1 
would  be  retained  in  the  absorbing  reagents,  nitrogen 
peroxide  is  readily  soluble  in  sulphuric  acid,  soda- 
lime,  or  potassium  hydroxide,  increasing  the  percent- 
age of  hydrogen  and  carbon  respectively.  Further- 
more, the  interstices  of  a  solid  reagent  such  as  cal- 
cium chloride  or  soda-lime  afford  an  excellent  oppor- 
tunity for  the  retention  of  oxygen  which  would  com- 
bine with  any  nitric  oxide  and  would  subsequently  be 
absorbed. 

Nitrogenous  substances  may  be  subdivided  into  two 
classes :  one  in  which  the  nitrogen  is  attached  to  an 

1  Nettlefold :  Chem.  News,  55,  28  ;  Russell  and  I^apraik :  J.  Chem.  Soc., 
2,  28  (1877)  ;  IJmich :  Monatshefte,  13,  90:  L,unge  :  Ber.  d.  chem.  Ges.,  18,  1391. 


COMBUSTION   OF   NITROGENOUS   SUBSTANCES          59 

oxygen  atom,  and  the  other  in  which  no  oxygen  is 
connected  with  the  nitrogen.  To  the  first  class  belong 
the  nitro,  nitroso,  isonitroso,  and  azoxy  bodies, 
oximes,  etc.,  while  the  second  class  includes  practi- 
cally all  other  nitrogenous  organic  com  pounds;  amines, 
amides,  nitriles,  etc. 

It  is  only  in  dealing  with  bodies  of  the  first  class 
that  any  modification  in  the  method  of  combustion  is 
necessary.  The  unoxidized  nitrogenous  bodies  are 
burned  exactly  as  described  on  page  52.  The  oxidized 
nitrogenous  compounds  when  heated  are  especially 
prone  to  liberate  oxides  of  nitrogen  and  some  provision 
is  necessary  to  eliminate  any  possible  effect  of  their 
presence  on  the  final  results.  These  precautions  con- 
sist of  one  of  two  essentially  different  operations.  The 
oxides  formed  are  absorbed  by  lead  peroxide,1  manga- 
nese dioxide,2  potassium  chromate,3  or  metallic  silver4 
or  more  commonly  they  are  reduced  by  metallic  cop- 
per.5 The  absorption  of  the  oxides  is  almost  always 
adapted  to  special  methods  and  is  open  to  grave  ob- 
jections. 

The  reduction  by  metallic  copper  is  by  far  the  sim- 
plest and  most  satisfactory  operation.  The  metallic 
copper  may  be  reduced  in  the  form  of  a  spiral,  by 
hydrogen,  methyl  or  ethyl  alcohol,  formic  acid,  or 

1  Kopfer:  Ztschr.  anal.  Chem.  (1878),  28. 

2  Perkin  :  J.  Chem.  Soc.,  (1880),  457  ;  Ber.  d.  chem.  Ges.,  13,  581. 

3  Perkin :  J.  Chem.  Soc.,  37,  i?i. 

4  Dennstedt :  Gazz.  chim.  ital.,  28,  78. 

5  The  use  of  a  copper  spiral  to  reduce  oxides  of  nitrogen  has  recently  been 
investigated  by  Klingemann  :  Ber.   d.   chem.   Ges.,  22,  3064;  Tower:  J.   Am. 
Chem.  Soc.,  21,  596 ;  Benedict :  Am.  Chem.  J.,  23,  334. 


60  ELEMENTARY   ORGANIC    ANALYSIS 

coal  gas,  and  after  cooling  introduced  into  the  exit 
end  of  the  combustion  tube. 

Another  method  for  the  decomposition  of  the  oxides 
of  nitrogen  consists  in  reducing  a  quantity  of  cupric 
oxide  in  close  proximity  to  the  boat  by  charring  or 
vaporizing  a  known  weight  (50  to  100  milligrams)  of 
chemically  pure  sucrose,  benzoic  acid,  or  naphthalene 
in  the  tube.  The  metallic  copper  thus  reduced  is  heated 
strongly  and  the  material  to  be  burned  then  heated. 
As  the  oxides  of  nitrogen  pass  over  the  hot  copper 
they  are  completely  reduced. 

Of  these  two  methods  the  latter1  is  much  the  easier, 
requiring,  as  it  does,  no  change  in  the  method  of  filling 
the  combustion  tube.  The  reducing  material,  sucrose, 
benzoic  acid,  or  naphthalene,  must  be  perfectly  pure. 
Pulverized  rock  candy  from  which  the  strings  have 
been  separated,  furnishes  a  remarkably  pure  sucrose. 
It  is  unnecessary  to  dry  the  material  unless  it  is  pul- 
verized on  an  especially  damp  day.  It  is  preserved  in 
a  test-tube  closed  with  a  good  rubber  stopper  and  fur- 
nishes an  excellent  standard  material  for  making 
check  combustions.  Kahlbaum's  pure  benzoic  acid 
and  naphthalene  have  also  given  excellent  satisfaction 
as  reducing  materials  in  this  laboratory. 

The  material  to  be  analyzed  is  placed  in  a  boat 
leaving  a  free  space  about  a  centimeter  in  length  in 
the  forward  end.  The  greater  portion  of  the  sucrose, 
benzoic  acid,  or  naphthalene  is  placed  in  this  space  and 
the  remainder  sprinkled  over  the  top  of  the  layer  of 

1  Am.  Chem.  J.,  23,  343. 


COMBUSTION   OF   NITROGENOUS   SUBSTANCES          6 1 

substance.  The  end  of  the  boat  containing  the  sucrose 
is  first  inserted  in  the  combustion  tube  and  the  boat 
pushed  in  till  it  nearly  touches  the  asbestos  plug.  The 
boat  should  not  touch  the  cupric  oxide,  but  be  sepa- 
rated by  a  centimeter  layer  of  air.  After  heating  the 
spiral  in  the  anterior  end  of  the  tube  the  heat  is  brought 
toward  the  boat  from  the  middle  of  the  combustion 
furnace,  hence  the  cupric  oxide  becomes  thoroughly 
heated  before  the  end  of  the  boat  containing  the  re- 
ducing material  is  heated.  The  sucrose  melts  at  143° 
and  distils  towards  200°,  giving  off  empyreumatic 
vapors  which  reduce  a  portion  of  the  contiguous  cupric 
oxide  which  becomes  still  more  heated  as  the  flames 
are  turned  on.  The  melted  sucrose  often  mixes  with, 
or  possibly  partially  dissolves,  the  substance  to  be 
burned  and  when  the  sugar  finally  chars  there  is  a 
large  excess  of  carbon  to  aid  in  reducing  the  nitro 
group.  Furthermore,  the  hot  copper  may  produce  a 
decomposition  of  the  benzoic  acid  vapor  which  passes 
over  it. 

When  a  new  combustion  tube  is  used  the  presence 
of  reduced  copper  is  readily  seen  as  a  layer,  some  two 
or  three  centimeters  long,  appearing  in  front  of  the  boat 
while  the  end  of  the  rear  cupric  oxide  spiral,  inserted 
after  the  boat,  is  always  seen  to  be  partially  reduced. 

Few  nitro  bodies  are  decomposed  with  an  evolution 
of  oxides  of  nitrogen  below  the  temperatures  necessary 
to  secure  the  vaporization  of  benzoic  acid  or  naphtha- 
lene, or  the  dry  distillation  of  sucrose.  It  may  be 
necessary,  however,  in  some  cases  to  place  the  redu- 


62  ELEMENTARY   ORGANIC    ANALYSIS 

cing  material  in  a  small  copper  boat  a  little  ahead  of 
the  porcelain  boat  containing  the  material  to  be  ana- 
lyzed. In  this  case  the  copper  can  be  reduced  before 
the  material  is  heated.  Volatile  nitro  compounds  are 
vaporized  in  a  current  of  nitrogen  as  described  on 
page  75  and  conducted  over  copper  reduced  by  a  known 
weight  of  either  of  these  reducing  materials. 

In  determining  the  percentages  of  carbon  and  hydro- 
gen it  is  necessary  to  calculate  the  weight  of  water 
and  carbon  dioxide  resulting  from  the  combustion  of 
the  reducing  material  and  deduct  these  weights  from 
the  total  increments  of  the  tubes.  The  difference  rep- 
resents the  weight  resulting  from  the  combustion  of 
the  material  to  be  analyzed.  By  multiplying  the 
weight  of  material  used  by  the  corresponding  factors1 
the  weight  of  water  and  carbon  dioxide  is  readily 
calculated. 

Sucrose  and  benzoic  acid  require  no  special  care  in 
weighing  and  burning,  but  naphthalene,  owing  to  its 
volatility,  necessitates  rapid  weighing  and  introduc- 
tion to  avoid  loss  from  vaporization.  Of  these  three 
substances  naphthalene  is  much  the  best  reducing  agent, 
weight  for  weight,  benzoic  acid  next,  and  sucrose  last. 
Unless  the  material  is  very  volatile,  sucrose  will  be 
found  to  give  most  excellent  results. 

The  most  familiar  method  of  reducing  oxides  of  nitrogen 
is  that  requiring  the  introduction  of  a  reduced  copper  spiral 
in  the  exit  end  of  the  combustion  tube.  A  ten  centimeter 
layer  of  cupric  oxide  is  removed  from  this  end  of  the  tube  and 
a  reduced  copper  spiral  of  equal  length  introduced.  Where 

1  .See  Appendix,  p.  82. 


COMBUSTION   OF   NITROGENOUS   SUBSTANCES          63 

a  number  of  combustions  of  different  materials  are  to  be  made 
two  tubes  may  be  held  prepared,  one  with  the  spiral  and  the 
other  filled  as  described  on  page  27.  If  the  last  ten  centi- 
meters of  the  layer  of  cupric  oxide  are  replaced  by  a  coil  pre- 
pared of  stout  copper  wire  thoroughly  oxidized,  the  tube  may 
be  used  for  the  combustion  of  oxidized  nitrogen  bodies  by 
simply  withdrawing  the  cupric  oxide  spiral  and  inserting  a 
reduced  copper  spiral. 

The  general  process  of  the  combustion  does  not  differ  ma- 
terially from  that  described  on  page  52.  In  order  to  prevent 
oxidation  and  consequent  diminution  in  reducing  power  of  the 
reduced  spiral  it  is  advisable  to  expel  the  oxygen  in  the  combus- 
tion tube  by  air1  before  introducing  the  spiral  or  connecting  the 
absorbing  system.  The  U-tube  used  to  purify  the  air  drawn 
through  the  absorbers  after  a  combustion  is  connected  directly 
with  the  stopper  in  the  entrance  end  of  the  combustion  tube. 
A  gentle  suction  by  the  mouth  or  an  aspirator  is  applied 
through  a  rubber  tube  connected  with  the  stopper  in  the  exit 
end  of  the  combustion  tube  and  sufficient  air  is  slowly  drawn 
through  the  tube  to  remove  all  oxygen.  The  reduced  spiral 
is  then  introduced,  the  absorbers  connected,  the  boat  inserted, 
and  the  combustion  carried  out  as  previously  described.  As 
soon  as  the  oxygen  is  admitted  the  burners  under  the  copper 
spiral  should  be  turned  off.  When  the  oxygen  is  escaping 
from  the  last  tube  of  the  absorbing  system  the  combustion  is 
at  an  end  and,  after  five  minutes  to  insure  removal  of  all 
traces  of  moisture  and  carbon  dioxide,  the  absorbers  may  be 
removed. 

The  copper  will,  on  being  heated,  absorb  the  oxygen  in  the 
air  in  the  tube  and  consequently  cause  a  back  suction.  While 
this  can  be  partly  counteracted  by  admitting  a  very  gentle 
current  of  oxygen  through  the  glass  stop-cock,  it  is  advisa- 
ble to  attach  an  unweighed  calcium  chloride  tube  to  the  final 
tube  in  the  absorbing  train  to  prevent  the  entrance  of  mois- 

1  Tower  :  J.  Am.  Chetn.  Soc.,  21,  597. 


64  ELEMENTARY   ORGANIC   ANALYSIS 

ture.  The  small  calcium  chloride  tube  used  in  the  end  of  the 
combustion  tube  when  cool  can  here  be  used  to  advantage. 

The  copper  spirals  prepared,  as  described  on  page  25,  are 
best  reduced  by  heating  strongly  in  a  Bunsen  flame  and  then 
thrusting  into  a  large  test-tube  containing  about  one  cubic 
centimeter  of  ethyl,  or  better  methyl,  alcohol.  The  hot  copper 
vaporizes  the  alcohol  and  the  cupric  oxide  is  immediately  re- 
duced. To  prevent  reoxidation  a  good  rubber  stopper  is  in- 
serted in  the  mouth  of  the  test-tube  as  soon  as  the  liberation 
of  gases  has  ceased.  The  cork  may  be  laid  loosely  in  the 
mouth  of  the  tube  and  firmly  inserted  as  soon  as  possible. 
The  spiral,  after  cooling,  is  freed  from  adhering  vapors  by 
placing  it  in  a  vacuum  desiccator  for  several  hours.  It  may 
also  be  dried  in  a  current  of  carbon  dioxide,  or  even  in  an  air- 
bath,  at  100°. 

The  preparation  of  the  reduced  copper  spiral  and  the  sub- 
sequent removal  of  the  reducing  agent  have  been  the  subject 
of  much  discussion.1  The  method  described  has  given  sat- 
isfaction. 

Many  nitro  bodies  can  be  burned  very  satisfactorily 
by  simple  admixture  with  three  or  four  volumes  of 
pure  dry  powdered  silica  such  as  is  used  in  glass  works. 
The  mixture  is  placed  in  a  porcelain  boat  and  the 
combustion  carried  out  as  described  on  page  52.  The 
sand  renders  the  substance  non-explosive,  and  a  regu- 
lar combustion  takes  place. 

If  the  absorbing  system  (Fig.  7)  is  used  the  oxides 
of  nitrogen  will  almost  invariably  be  retained  by  the 
sulphuric  acid  and  consequently  the  percentage  of 
hydrogen  alone  will  be  affected.  If  the  substance 

1  Neumann  :  Monatshefte,  13,  40  ;  Johnson  :  Chem.  News,  67,  99  ;  Thudi- 
cum  and  Hake  :  Ibid.,  33,  218  ;  Weyl  :  Ber.  d.  chem.  Ges.,  15,  1139 ;  I^ietzen- 
mayer  :  Ibid.,  11,  306  ;  Schwarz  :  Ibid.,  13,  559  ;  Erdmann  :  J.  prakt.  Chem.,  76, 
96  ;  Schrotter :  Ibid.,  76,  480 ;  I^autemann  :  Ann.  Chem.  (I^iebig),  109,  301; 
Ritthausen  :  Ztschr.  anal.  Chem.,  18,  602. 


BODIES   CONTAINING  THE   HALOGENS  65 

burns  with  explosive  violence  and  the  gas  bubbles 
through  the  sulphuric  acid  tube  faster  than  one  bub- 
ble per  second  a  portion  of  the  oxides  of  nitrogen  may 
be  carried  over  into  the  soda-lime.  When  the  percent- 
age of  hydrogen  is  not  especially  desired  the  admixture 
with  pure  silica  will  often  be  found  a  very  simple  and 
rapid  method  of  determining  the  percentage  of  carbon 
and  in  most  cases  the'  percentage  of  hydrogen  will  not 
be  excessively  high. 

COMBUSTION  OF  BODIES  CONTAINING  THE 
HALOGENS 

When  compounds  containing  chlorine,  bromine,  or 
iodine  are  burned  in  the  manner  described  on  page  52 
the  corresponding  cuprous  haloid  is  formed.  The 
volatility  of  the  halogen  compounds  of  copper  renders 
it  almost  impossible  to  prevent  their  sublimation  into 
the  water-absorbing  tube.  Furthermore,  at  a  high 
temperature  and  in  the  presence  of  oxygen  they  easily 
lose  a  portion  of  their  halogen. 

The  retention  of  the  halogens  is  often  effected  by  introdu- 
cing a  reduced  copper  spiral,1  ten  centimeters  long,  in  the  exit 
end  of  the  combustion  tube,  removing  a  ten  centimeter  length 
of  the  cupric  oxide  for  the  purpose,  exactly  as  was  done  in 
reducing  the  oxides  of  nitrogen,  as  described  on  page  62.  The 
halogens  are  retained  by  this  spiral  which  is  heated  only  to 
a  very  low  red  heat,  care  being  taken  not  to  volatilize  the 
cuprous  haloid.  During  the  final  combustion  in  oxygen  gas 
the  spiral  must  be  cooled  to  prevent  interaction  of  the  haloid 
and  the  oxygen. 

1  Glaser:  Ann.  Chem.  (Iviebig),  Suppl.,  7,  213;  Stadler :  Ibid.,  69,  335. 
5 


66  ELEMENTARY   ORGANIC    ANALYSIS 

The  great  objection  to  the  retention  of  the  halogens  as  cop- 
per haloids  is  their  ready  action  with  oxygen.  Vo'lcker1  sug- 
gests a  mixture  of  cupric  oxide  and  lead  oxide  in  which  the 
halogen  is  converted  to  the  non-volatile  lead  compound. 
The  basicity  of  lead  oxide,  however,  renders  it  liable  to  ab- 
sorb carbon  dioxide.2 

By  using  metallic  silver  instead  of  copper,  halogen  com- 
pounds are  obtained  unacted  upon  by  oxygen  and  withstand- 
ing a  high  heat ;  i.  e.,  non-volatile.  Silver3  in  the  lace,  mo- 
lecular, foil,  or  spiral  form  when  heated  combines  readily 
with  the  various  halogens. 

In  using  a  silver  spiral  the  combustion  tube  is  filled  as  de- 
scribed on  page  27,  removing  a  ten  centimeter  length  of  cu- 
pric oxide  from  the  exit  end  to  make  a  place  for  a  similar 
layer  of  metallic  silver  in  either  of  the  above-mentioned  forms. 
All  oil  or  organic  matter  should  be  burned  off  in  a  prelimi- 
nary heating  which  can  be  effected  in  a  current  of  oxygen  since 
silver  is  not  acted  upon  by  oxygen.  The  silver  halogen  com- 
pounds, if  formed  in  large  quantities,  are  liable  to  fuse  the 
silver  to  the  glass.  By  removing  the  silver  and  heating  it 
in  a  current  of  pure  hydrogen  the  haloid  is  completely  re- 
duced to  the  metal  which  can  be  used  repeatedly. 

Moissan4  recommends  burning  bodies  containing  fluorine 
in  a  copper  tube  filled  with  eighty  parts  of  cupric  oxide  and 
twenty  parts  of  litharge  which  retains  the  fluorine  liberated. 

The  most  satisfactory  method  of  burning  bodies 
containing  halogens  involves  the  use  of  lead  chromate 
instead  of  cupric  oxide  as  the  oxidizing  agent.  The 
tube  is  filled  precisely  as  when  the  latter  reagent  is 
used  (p.  27).  The  cupric  oxide  spiral  in  the  anterior 
part  of  the  tube  is  likewise  inserted.  Since  lead  chro- 

1  Chem.  Gaz.  (1849),  245. 

2  Kopfer :  Ztschr.  anal.  Chem.  (1878),  30. 

3  Stein  :  Ibid.,  8,  83  ;  Kraut':  Ibid.,  2,  242. 

4  Compt.  rend.,  107,  992. 


BODIES   CONTAINING   THE   HALOGENS  67 

mate1  is  hygroscopic  it  is  necessary  to  burn  out  the 
tube  in  a  current  of  oxygen  as  described  on  page  45. 
The  tube  should  not  be  heated  enough  to  melt  the 
chromate.  The  halogen  in  the  substance  burned  com- 
bines with  the  lead,  forming  the  corresponding  lead 
haloid.  These  haloids  are  somewhat  volatile  and  it  is 
important  that  the  last  ten  centimeters  of  the  layer  of 
chromate  should  be  heated  only  high  enough  to  pre- 
vent the  condensation  of  moisture.  The  sublimed 
haloids  condense  on  the  cooler  portions  of  the  chro- 
mate and  are  not  carried  along  to  the  water-absorbing 
tube.  The  combustion  is  finished  as  before  in  a  cur- 
rent of  oxygen  which  does  not,  however,  regenerate 
the  oxidizing  agent  as  in  the  case  of  reduced  copper 
oxide,  hence  it  is  necessary  to  refill  the  combustion 
tube  with  fresh  material. 

The  complete  retention  o,f  the  halogens  is  at  times  attended 
with  great  difficulty  as  the  following  instances  show : 

Meyer  and  Wachter2  found  that  in  burning  iodosobenzoic 
acid  the  halogen  was  not  retained  by  lead  chromate  even 
with  the  addition  of  several  long  silver  spirals. 

Mauthner  and  Suida3  found  too  large  a  percentage  of  car- 
bon when  burning  tribromacrylic  acid  even  when  using  lead 
chromate  and  a  silver  spiral. 

The  low  fusibility  of  lead  chromate  has  led  to  several  sug- 
gestions for  a  substitute  of  higher  fusing-point.  L,iebig4 
recommends  a  mixture  of  lead  and  potassium  chromates. 

By  mixing  one  part  of  red  lead  with  four  parts  of  finely 

1  Erdmann  :  J.  prakt.  Chem.,  81,  180 ;  Bruce- Warren  :  Chem.  News,  71,  143. 

2  Ber.  d.  chem.  Ges.,  25,  2632. 

3  Mpnatshefte  (1881),  p.  in. 

4  Anleit.  z.  Anal,  organ.  Korper,  p.  32. 


68  ELEMENTARY   ORGANIC    ANALYSIS 

powdered  lead  chromate  de  Roode1  obtains  a  mixture  which 
is  at  least  less  fusible  and  has  much  greater  surface  for  action. 

This  mixture  was  found  especially  serviceable  by  Remsen 
in  burning  the  chloride  of  orthosulphobenzoic  acid  and  allied 
bodies.2  It  was  necessary,  however,  to  mix  the  substance 
with  powdered  potassium  dichromate. 

In  burning  compounds  containing  boron  and  fluorine  Lan- 
dolph3  found  that  a  ten  centimeter  layer  of  lead  chromate,  if 
not  heated  too  high,  retained  all  boron  and  fluorine. 

BODIES  CONTAINING  SULPHUR 

When  bodies  containing  sulphur  are  burned  in  the 
ordinary  way  with  cupric  oxide,  cupric  sulphate  is 
first  formed  and  is  then  decomposed  by  a  high  heat, 
liberating  sulphur  dioxide4  which  is  absorbed  in  the 
carbon  dioxide  tube. 

The  retention  of  the  oxides  of  sulphur  may  be  accomplished 
at  the  expense  of  the  hydrogen  determination  by  mixing  a 
small  quantity  of  chromic  oxide  or  potassium  dichromate 
with  the  sulphuric  acid  in  the  water-absorbing  tube.  All 
sulphur  dioxide  is  retained.  Winkler5  suggests  a  determina- 
tion of  the  sulphur  present  by  titration  of  the  unreduced 
chromic  acid. 

The  sulphur  dioxide  may  also  be  absorbed  by  a  layer  of 
pure  lead  dioxide6  placed  in  the  exit  end  of  the  combustion 
tube  and  warmed  to  150°  or  180°  to  prevent  the  condensation 
of  moisture.  The  combustion  tube  must  be  somewhat  longer 
than  that  usually  employed  and  so  filled  that  a  ten  centi- 

1  Am.  Chem.  J.,  12,  226. 
-  Ibid.,  1 8,  803. 

3  Ber.  d.  chem.  Ges.,  12,  1586. 

4  In  the  case  of  substances  containing  iron,  sulphur  trioxide  is  also  liber- 
ated.    Ztschr.  anal.  Chem.,  14,  16. 

&  Ibid.,  21,  545  (1882). 

6  Warren  :  Am.  J.  Sci.,  2nd  ser.  41,  40 ;  Kopfer  :  Ztschr.  anal.  Chem.  (1878), 
28. 


BODIES   CONTAINING   SULPHUR  69 

meter  layer  of  lead  oxide  is  in  that  portion  of  the  tube  protru- 
ding from  the  furnace.  The  rest  of  the  tube  is  filled  as  de- 
scribed on  page  27.  The  layer  of  lead  oxide  is  kept  warm  by 
a  small  air-bath  or  sheet  iron  hood.  All  the  sulphur  dioxide 
is  retained  as  lead  sulphate. 

Pb02  +  S02  -  PbS04. 

With  pure,  specially  prepared  lead  dioxide  there  is  no  danger 
of  the  retention  of  carbon  dioxide  though  if  lead  monoxide  is 
present  some  of  the  gas  will  be  absorbed. 

Precipitated  manganic  peroxide  mixed  with  a  concentrated 
solution  of  potassium  dichromate  containing  one-tenth  of 
its  weight  of  potassium  chromate  and  dried  forms  a 
mixture  that  Perkin1  uses  in  place  of  lead  dioxide  to  retain 
sulphur  dioxide  and  nitrogen  peroxide.  The  mixture  is 
placed  in  the  exit  end  of  the  combustion  tube  and  is  main- 
tained at  a  temperature  of  200°  to  250°  by  means  of  a  small 
air-bath.  The  material  is  regenerated  by  heating  strongly 
in  a  current  of  oxygen. 

In  burning  bodies  containing  sulphur  it  is  better  to 
use  a  combustion  tube  filled  with  lead  chromate  or  the 
mixture  of  de  Roode  described  on  page  68.  The  sul- 
phur dioxide  is  all  retained  as  lead  sulphate,  and  if  a 
ten  centimeter  layer  of  the  lead  chromate  nearest  the 
exit  end  of  the  combustion  tube  is  not  too  ^strongly 
heated  the  lead  sulphate  will  condense  there  and  not 
pass  into  the  absorbing  apparatus.  The  manipulation 
is  identical  with  that  given  for  the  combustion  of  sub- 
stances containing  halogens. 

Volatile  compounds  containing  sulphur  and  nitrogen  must 
be  burned  very  slowly  as  otherwise  sulphur  dioxide  is  liable 
to  escape.2 

1  Ztschr.  anal.  Chem.,  ai,  273. 

*  V.  Meyer  and  Stadler :  Ber.  d.  chem.  Ges.,  17,  1577. 


70  ELEMENTARY   ORGANIC   ANALYSIS 

BODIES  CONTAINING  THE  ALKALI  METALS 

Compounds  of  this  class  when  burned  retain  in  the 
ash  material  quantities  of  carbon  dioxide  which  should 
either  be  determined  and  added  to  the  carbon  dioxide 
collected  in  the  absorbing  system  or  the  substance  to 
be  burned  should  be  mixed  with  some  material  such 
as  potassium  dichromate,1  chromic  oxide,2  cupric  phos- 
phate,3 tungstic4  and  silicic5  acids  to  expel  the  com- 
bined carbon  dioxide. 

Lead  chromate  with  an  admixture  of  one-tenth  its 
weight  of  potassium  dichromate  is  the  best  reagent 
for  this  purpose. 

If  a  calcium  salt  is  being  burned  Leiben  and  Zeisel6  sug- 
gest igniting  the  boat  and  ash  in  the  blast  to  drive  off  the 
carbon  dioxide  which  is  determined  by  the  loss  in  weight. 
To  prevent  contamination  from  adhering  cupric  oxide,  the 
boat  is  introduced  into  the  combustion  tube  in  a  cylinder  of 
platinum  foil. 

DIFFICULTLY  COMBUSTIBLE  BODIES 

Much  difficulty  is  often  experienced  in  securing  de- 
terminations of  carbon  and  hydrogen  in  bodies  which 
are  difficult  to  oxidize  completely.  In  such  bodies 
either  the  carbonaceous  residue  is  of  a  very  resistant 
character  or  the  gases  given  off  in  the  decomposition 
of  the  molecule,  such  as  carbon  monoxide,  methane, 
aldehyde,  or  acetylene,  are  not  easily  oxidized  by  the 
cupric  oxide. 

"Wlslicenus  :  Ann.  Chem.  (lyiebig),  166,  13. 

Schwarz  and  Pastrovich  :  Ber.  d.  chem.  Ges.,  13,  1641. 

De  Chaubray  :  Compt.  rend.  (1842),  i,  645. 

Cloez:  Jahresberichte,  1864. 

Schaller:  Bull.  Soc.  Chim.,  2,  414. 

Monatshefte,  4,  27. 


DIFFICULTLY   COMBUSTIBLE    BODIES  71 

Few  substances  will  not  be  completely  burned  when 
the  combustion  is  carried  out  in  a  current  of  oxygen 
(p.  49).  Certain  bodies  containing  nitrogen,  halogens, 
or  sulphur  require  special  precautions  in  burning  to 
effect  a  complete  oxidation  of  the  carbonaceous  residue. 

Hippuric  acid  is  not  readily  burned  in  a  porcelain 
boat,  but  when  a  copper  (cupric  oxide)  boat  is  used 
the  combustion  is  complete.  The  copper  boat  should 
have  a  cover  (p.  30). 

Malonic  nitrile,1  containing  as  it  does  about  forty-two  per 
cent,  of  nitrogen,  leaves  a  very  resistant  residue. 

A  platinum  boat,  owing  to  the  condensation  of  the 
oxygen  on  the  metal,  facilitates  combustion.  This 
property  of  platinum  may  also  be  made  use  of  by  cov- 
ering the  material  in  the  boat  with  three  or  four  times 
its  volume  of  well  ignited  platinum  sponge.2 

Of  the  oxidizing  mixtures  finely-powdered  cupric 
oxide  is  especially  effective.  Owing  to  its  hygro- 
scopic nature  it  is  important  to  ignite  it  thoroughly  in 
a  crucible  and  allow  it  to  cool  in  a  desiccator  before 
using  in  the  combustion  tube.  At  times  the  material 
may  be  so  hard  to  burn  as  to  necessitate  mixing  with 
cupric  oxide  and  introducing  into  the  tube  in  the  form 
of  a  powder,  filling  the  space  usually  reserved  for  the 
boat.  In  such  cases  the  old  bayonet  form  of  tube3 
may  be  advantageously  used. 

Cyanogen  compounds4  are  incorporated  intimately 

1  Hesse  :  Am.  Chem.  J.,  18,  727. 

2  Demel :  Ber.  d.  chem.  Ges.,  15,  604. 

3  Meyer  and  Jacobsen  :  "  I^ehrbuch  der  organischen  Chemie,"  p.  15. 
•*  I4ppmann  and  Fleissner:  Monatshefte,  7,  9. 


72  ELEMENTARY   ORGANIC   ANALYSIS 

with  a  mixture  of  one  part  potassium  dichromate  and 
ten  parts  lead  chromate.  Powdered  lead  chromate  is 
also  hygroscopic  and  requires  special  drying  before 
use. 

A  layer  of  potassium  chlorate  may  be  fused  into  the 
boat  and  the  substance  then  added  and  weighed. 

Substances  which,  when  heated,  decompose  into  gases 
which  are  not  readily  combustible  must  be  burned 
with  a  very  long  layer  of  cupric  oxide  at  a  high  heat1 
and  at  a  very  slow  rate. 

Zincke2  found,  in  studying  the  derivatives  of  thelactone  of 
0-phenylglycerinecarbonic  acid,  that  satisfactory  results  could 
not  be  obtained  with  cupric  oxide.  The  carbon  was  always 
three  per  cent,  too  low.  By  using  lead  chromate,  however, 
he  obtained  the  theoretical  results.  The  decomposition  of 
these  bodies  forming  carbon  monoxide,  which  escapes  un- 
burned,  seemed  to  him  to  be  the  difficulty. 

The  special  methods  of  organic  analysis  relying  on 
a  short  layer  of  oxidizing  material  are  inadequate  for 
the  combustion  of  this  class  of  compounds  as  a  long 
layer  of  highly  heated  oxidizing  material  is  essential.3 

Zeisel4  found  that  Kopfer's  method  could  not  be  used  in 
analyzing  the  derivatives  of  the  methyl  ester  of  colchicein  as 
the  carbon  and  hydrogen  were,  owing  to  incomplete  oxida- 
tion, respectively  two  and  one  per  cent,  too  low. 

Herzigs  and  Skraup6  found  that  Kopfer's  method  could  not 
be  used  satisfactorily  with  ethyl  or  acetyl  quercetin  or  chin- 
ch oleupon  hydrochloride. 

Haber  and  Grinberg  :  Ztschr.  anal.  Chem.,  36,  558. 

Ber.  d.  chem.  Ges  ,  25,  408. 

Blau :  Monatshefte  (1889),  357;  Wislicenus:  Ann.  Chem.  (lyiebig),  242,  27. 

Monatshefte,  7,  573. 

Ibid.,  9,  540. 

Ibid.,  9,  807. 


LIQUIDS   AND   VOLATILE    BODIES  73 

Wegscheider1  found  that  the  true  esters  of  opianic  acid  can 
not  be  burned  with  cupric  oxide,  while  with  lead  chromate 
satisfactory  results  may  be  obtained. 

Among  other  cases  in  which  difficulty  was  experienced  are 
the  following : 

Claissen,2  in  burning  a  number  of  ethyl  esters,  found  insuffi- 
cient oxidation  of  the  liberated  gases. 

Skraup3  had  difficulty  in  burning  some  esters  in  the  mucic 
acid  series. 

Smith4  emphasizes  the  necessity  of  a  very  slow  combustion 
with  phenylimidocarbonicdiethylester,  owing  to  the  escape  of 
unoxidized  gases. 

Guareschi  and  Graude5  found  that  hydrocarbons  (methane 
and  ethylene)  were  formed  during  the  combustion  of  dicyan- 
methylhydroethyldioxypyridine. 

COMBUSTION  OF  LIQUIDS  AND  VOLATILE  BODIES 

Certain  modifications  in  the  method  of  weighing 
and  introducing  the  substance  are  necessary  in  burn- 
ing liquids  or  volatile  bodies. 

If  the  material  has  a  boiling-point  above  170°  C., 
it  may,  as  a  general  rule,  be  weighed  in  the  boat  as 
directed  for  solids.  It  is  best  to  weigh  the  boat  and 
material  just  before  using  and  transfer  immediately 
to  the  combustion  tube. 

Dudley6  uses  a  small  bottle,  through  the  cork  of  which  a 
bulb  pipette  or  medicine  dropper  is  thrust.  The  bottle  is 
weighed,  liquid  drawn  into  the  pipette,  delivered  into  the 
boat  and  the  bottle  again  weighed,  thereby  giving  the  weight 

I  Monatshefte,  14,313. 

*  Ber.  d.  chem.  Ges.,  35,  1768. 

3  Monatshefte,  14,  476. 

4  Am.  Chem.  J,,  16,  391. 

5  Rendiconti  Acad.  Torino,  33,  16. 

6  Ber.  d.  chem.  Ges.,  21,  3172. 


74  ELEMENTARY   ORGANIC  ANALYSIS 

of  liquid  used.  The  boat  should  be  immediately  thrust  into 
the  combustion  tube  as  the  weighing-bottle  need  not  be 
weighed  till  later. 

Reichardt1  recommends  weighing  heavy  oils,  not  too  hygro- 
scopic, by  dropping  them  on  a  layer  of  cupric  oxide  powder 
in  the  bottom  of  a  boat.  A  weighing-bottle  is  used  and  the 
amount  determined  by  difference. 

Liquids  boiling  from  100°  to  150°  may  be  weighed 
in  a  glass  tube,  sealed  at  one  end,  about  three  milli- 
meters internal  diameter  and  twenty-five  millimeters 
long.  The  tube  is  easily  filled  by  means  of  a  fine  pi- 
pette made  by  drawing  out  one  end  of  a  piece  of  tu- 
bing (five  millimeters  internal  diameter)  to  a  fine  jet. 
In  weighing,  it  is  advisable  to  bend,  a  piece  of  moder- 
ately stout  copper  wire  in  a  form  to  rest  on  the  bal- 
ance pan  and  support  the  glass  tube  in  an  upright 
position.  The  glass  tube  should  be  placed  in  the  boat 
on  a  layer  of  cupric  oxide  (to  prevent  fusion  of  the 
glass  to  the  porcelain)  with  its  open  end  directed  to- 
wards the  exit  end  of  the  combustion  tube  and  suffi- 
ciently raised  to  prevent  loss  of  liquid.  It  should  be 
previously  determined  if  the  boat  and  the  tube  can  be 
inserted  into  the  combustion  tube.  A  bent  piece  of 
sheet  copper  whose  surface  is  thoroughly  oxidized  may 
be  used  as  a  carrier  for  the  glass  tube.  The  rear  end 
of  the  tube  should  be  so  braced  that  it  cannot  slide 
back  and  cause  the  front  end  to  drop. 

Reichardt2  places  powdered  cupric  oxide  in  the  tube  to  ab- 
sorb the  liquid. 

A  small  loosely  fitting  stopper   (Fig.  13)  may   be 

1  Archiv.  der  Pharm.,  227,  640. 

2  I,oc.  cit. 


LIQUIDS   AND   VOLATILE   BODIES 


75 


made  by  softening  and  flattening  the  end  of  a  glass 
rod  of  a  diameter  to  fit  the  tube. 

Liquids  boiling  under  100°  are  best  vaporized  out- 


Fig.  13.  Fig.  14.  Fig.  15. 

side  the  tube1  and  the  vapor  driven  into  the  tube  by 
a  current  of  nitrogen,  air,  or  oxygen.  As  these  last 
two  form  explosive  mixtures  with  many  vapors  they 
cannot  often  be  used. 

A  simple  arrangement  for  vaporizing  such  liquids 
is  shown  in  Figure  15  and  consists  of  a  piece  of  glass 
tubing,  five  millimeters  internal  diameter,  which  is 
bent  in  a  U,  one  arm  of  which  is  again  bent  at  right 
angles  and  drawn  down  so  as  to  be  inserted  conve- 
niently in  the  stopper  in  the  entrance  end  of  the  com- 
bustion tube.  The  other  arm  is  left  in  a  vertical  po- 
sition. The  liquid  to  be  analyzed  is  drawn  into  a 
previously  weighed  piece  of  three  millimeter  glass  tu- 
bing drawn  out  at  both  ends.  The  ends  are  sealed 
by  fusion,  and  after  again  weighing,  the  tube  is  tightly 

1  Dudley  :  Ber.  d.  chem.  Ges.,  21,  3172. 


76  ELEMENTARY   ORGANIC   ANALYSIS 

inserted  in  a  short  piece  of  rubber  tubing  slipped  over 
the  open  arm  of  the  U-tube.  By  pressing  the  long 
point  on  the  side  of  the  U  the  end  is  broken  and  the 
liquid  will  drop  into  the  bend  of  the  U.  To  remove 
all  liquid  a' rubber  tube  conducting  the  purified  and 
dried  nitrogen  (air  or  oxygen)  is  slipped  over  the  other 
end  of  the  drawn-out  tube  and  the  slender  tip  broken 
by  pressing  with  a  pair  of  pinchers.  A  very  slow 
current  of  the  gas  is  conducted  through  the  tube,  and 
by  gently  warming  the  U-tube  (by  immersing  it  in  a 
beaker  of  warm  water  for  example)  the  liquid  is  va- 
porized and  driven  into  the  combustion  tube  at  any 
desired  rate.  By  immersing  the  U-tube  in  cold  water 
or  a  freezing-mixture,  liquids  of  very  low  boiling- 
points  can  be  very  readily  burned,  as  their  vaporiza- 
tion can  be  easily  controlled  by  raising  or  lowering 
the  beaker  containing  the  freezing-mixture.  When 
nitrogen  is  used,  as  is  generally  the  case,  the  gas  freed 
from  carbon  dioxide  and  moisture  should  previously 
be  conducted  through  the  small  U-tube  and  the  com- 
bustion tube  until  all  air  is  expelled.  Under  these 
circumstances  the  light  hydrocarbons,  ether,  etc.,  can 
be  vaporized  in  a  current  of  nitrogen  with  no  danger 
of  an  explosion. 

Nitrogen  for  this  operation  is  best  prepared  as  fol- 
lows :  A  current  of  air  is  caused  to  bubble  through 
strongest  ammonium  hydroxide  and  then  to  pass  over 
a  heated  copper  spiral.  The  copper  undergoes  alter- 
nate oxidation  (by  the  oxygen  of  the  air)  and  reduc- 
tion (by  the  ammonia),  yielding  nitrogen  containing 


LIQUIDS   AND   VOLATILE   BODIES  77 

an  excess  of  ammonia  which  is  removed  by  bubbling 
through  sulphuric  acid.  The  gas  is  held  in  some 
form  of  gasometer  (p.  6). 

One  hundred  and  fifty  cc.  of  strongest  ammonium  hydrox- 
ide are  placed  in  a  two-necked  Woulff  bottle  or  a  wide- 
mouthed  bottle  fitted  with  a  two-holed  rubber  stopper.  Air 
is  forced  or  drawn  through  a  glass  tube,  dipping  under  the 
surface  of  the  ammonium  hydroxide  and  the  air  containing 
a  considerable  quantity  of  ammonia  gas  is  conducted  over  a 
ten  centimeter  copper  spiral  (page  25),  (preferably  constructed 
of  stout  wire),  which  is  heated  in  a  thirty  centimeter  length 
of  Jena  glass  combustion  tubing.  A  good  Bunsen  burner 
with  a  fish-tail  top  will  give  enough  heat. 

The  gas  is  then  conducted  through  a  gas  washing-bottle 
containing  thirty  per  cent,  sulphuric  acid,  to  which  a  few 
drops  of  litmus  solution  have  been  added.  The  gas  freed 
from  the  excess  of  ammonia  is  conducted  into  the  gasometer. 
The  air  current  may  be  maintained  in  many  ways.  A  water- 
blast,  if  accessible,  is  the  most  convenient  form  of  apparatus 
for  this  purpose. 

If  all  the  connections  between  the  various  parts  of  the  ap- 
paratus are  tight  the  air  may  be  drawn  through  the  system 
by  suction  either  from  the  valve  C  in  the  Pepy  gasometer 
(Fig.  2)  or  its  corresponding  valve  on  the  Mitscherlich 
gasometer.  A  rubber  tube  connects  the  gas  washing-bottle 
containing  the  sulphuric  acid  and  litmus  solution  directly 
with  valve  C.  The  orifice  at  the  base  of  the  gasometer  is 
opened  and  the  valve  C  slowly  turned.  As  water  rushes 
out  at  the  bottom,  air  is  drawn  in  at  the  top.  After  drawing 
a  liter  of  gas  through  the  system  and  displacing  all  air  by 
nitrogen  the  gasometer  should  be  filled  again  and  the  opera- 
tion repeated.  The  gas  now  entering  the  gasometer  is  pure 
nitrogen  and  the  vessel  may  be  completely  filled. 

A  filter-pump  having  a  long  rubber  tube  attached  to  its 


78  BlyEMKNTARY   ORGANIC   ANALYSIS 

exit  tube  may  also  advantageously  be  used  to  fill  the  gasom- 
eter with  nitrogen.  A  current  of  air  is  drawn  through  the 
apparatus  by  means  of  the  pump  and  the  gas  mixed  with 
water  is  allowed  to  flow  into  the  opening  at  the  base  of  the 
gasometer.  The  air  current  should  be  maintained  for  several 
minutes  before  the  gas  is  finally  collected. 

By  observing  the  color  of  the  copper  spiral  the  operation 
can  be  well  regulated.  As  there  should  always  be  an  excess 
of  ammonia  gas  in  the  air,  all  the  spiral  but  the  front  end 
should  be  in  the  reduced  state.  As  soon  as  it  begins  to  oxi- 
dize for  any  distance  it  is  an  indication  that  the  supply  of 
ammonia  is  becoming  exhausted  and  the  bottle  containing  the 
ammonium  hydroxide  must  either  be  warmed  by  immersion 
in  a  crystallizing  dish  of  hot  water  or  the  air  current  must  be 
stopped  and  the  ammonium  hydroxide  renewed.  A  few  pre- 
liminary trials  should  be  made.  No  difficulty  will  be  expe- 
rienced in  preparing  from  fifteen  to  twenty  liters  of  nitrogen 
with  the  amount  of  ammonium  hydroxide  used.  If  the  sul- 
phuric acid  turns  blue  a  fresh  supply  of  acid  must  be  added. 

The  gas  is  purified  by  passing  it  from  the  gasometer 
through  any  of  the  purifying  systems,  though  sulphuric  acid 
should  be  used  as  a  drying  agent  to  remove  the  last  traces  of 
ammonia  gas. 

Warren1  and  Kassner2  vaporize  volatile  liquids  from  small 
capillary  bulbs  in  an  arm  of  the  combustion  tube  extending 
outside  the  furnace  and  slanting  upward. 

Low  boiling  liquids  may  be  sealed  in  small  bulbs 
with  capillary  openings,  introduced  into  the  combus- 
tion furnace,  and  there  vaporized. 

A  small  bulb  holding  from  three-  to  four-tenths  of  a 
cubic  centimeter  is  blown  on  the  end  of  a  tube  which 
has  been  drawn  out  to  a  capillary  (Fig.  14)  and  the 

1  Am.  J.  Sci.,  2nd  series,  38,  387. 
-  Ztschr.  anal.  Chem.,  26,  585. 


LIQUIDS   AND   VOLATILE   BODIKS  79 

bulb  tube  weighed.  By  warming  the  bulb  to  expel 
the  air,  dipping  the  end  of  the  capillary  in  the  liquid 
and  cooling  the  bulb,  sufficient  liquid  may  be  drawn 
up  to  fill  it  nearly.  After  driving  all  liquid  out  of  the 
capillary  the  bulb  is  finally  sealed.  Just  before  intro- 
ducing the  bulb  into  the  combustion  tube  a  file  scratch 
is  made  a  few  millimeters  from  the  end  of  the  capil- 
lary, and,  holding  the  tube  by  the  capillary  rather  than 
by  the  bulb,  the  tip  is  broken  off.  The  bulb  is  laid 
in  a  boat  with  the  open  end  pointing  towards  the  exit 
end  of  the  combustion  tube  and  the  boat  instantly 
thrust  into  the  tube.  The  relative  size  of  bulb  and 
boat  should  be  previously  determined  to  see  if  the  two 
can  be  inserted  in  the  tube.  Also  provision  should 
be  made  to  prevent  the  bulb  from  sliding  back,  there- 
by lowering  the  capillary  tube.  The  criticism  has 
been  made  that  a  certain  residuum  of  gas  remains  un- 
oxidized  in  the  bulb,  but  nevertheless  this  method  is, 
and  has  been,  almost  universally  used  for  liquids  of 
low  boiling-point. 

A  longer  capillary,1  with  or  without  a  bulb,2  facilitates  in 
the  slow  evaporation  of  liquids  if  the  heat  is  inadvertently 
carried  above  the  boiling-point. 

Hempel3  seals  the  end  of  the  long  capillary  with  a  fusible 
alloy  of  Wood 's  metal  ten  parts  and  mercury  two  to  three 
parts  which,  on  solidifying,  does  not  expand  and  break  the 
glass.  By  gentle  heat  the  metal  is  melted  and  the  capillary 
is  opened. 

1  Kopfer  (Ztschr.   anal.   Chem.,    17,    15)   used  a  capillary  six  centimeters 
long. 

2  Zulkowsky  (Monatshefte,  6,  450)   used  a  two   millimeter  tube  thirteen 
centimeters  long. 

3  Ztschr.  anal.  Chem.,  17,  414. 


80  ELEMENTARY   ORGANIC   ANALYSIS 

COMBUSTION  OF  EXPLOSIVE  BODIES 

Compounds,  such  as  diazo  and  nitro  bodies,  that 
explode  on  heating  must  be  mixed  or  diluted  with 
some  inert  material  such  as  powdered  lead  chromate, 
cupric  oxide,  or  quartz. 

Fine  cupric  oxide1  is  mixed  with  the  material  which 
is  best  placed  in  a  long  copper  boat.  At  times  the 
mixture  is  separated  by  dams  of  pure  cupric  oxide  to 
isolate  small  portions.2 

Picric  acid  and  picramid  and  allied  bodies  are  easily 
burned  when  mixed  with  three  or  four  volumes  of 
finely  powdered  quartz.3 

For  the  combustion  of  the  more  explosive  compounds  such 
as  nitroglycerine,  etc.,  recourse  must  be  had  to  one  of  the 
special  methods  ;  for  instance,  that  of  Hempel,4  of  burning 
in  a  vacuum. 

CALCULATION  OF  RESULTS 

As  three-elevenths  of  the  carbon  dioxide  is  carbon 
the  per  cent,  of  this  element  is  found  by  means  of  the 

formula  : 

wt.  carbon  dioxide  X  ^oo 

Per  cent,  carbon  =  ^ ^—  • 

wt.  substance  X  n 

As  one-ninth  of  the  water  is  hydrogen  the  per  cent, 
of  this  element  is  found  by  means  of  the  formula : 

wt.  water  X  100 


Per  cent,  hydrogen  = 


wt.  subvStance  X  9 


1  Schwarz :  Ber.  d.  chem.  Ges.,   13,  559;  Janowsky  :  Monatshefte,  6,  462; 
Ibid.,  9,  836  ;  Kder  :  Ber.  d.  chem.  Ges.,  13,  172. 

2  Jackson  and  I^amar  (Am.  Chem.  J.,  18,  676),  in  burning  dinitrophloroglu- 
cindiethylester,  found  this  procedure  necessary. 

8  Am.  Chem.  J.,  23,  346. 

*  Ztschr.  anal.  Chem.,  17,  414. 


CALCULATION   OF   RESULTS  8 1 

The  logarithmic  calculations,  using  the  factors  given 
in  the  appendix,  are  as  follows : 
Log.  wt.  H2OX  ioo  =  Log.  wt.  CO2X 100  = 

Log.  factor  =  1.04883      Log.  factor  =  1.43573 

Colog.  wt.  subs.     =  Colog.  wt.  subs.     = 

Log.  per  cent.  H     =  Log.  per-  cent.  C     = 

Where  known  weights  of  sucrose,  benzoic  acid,  or 
naphthalene  are  used  in  the  tube  to  effect  reduction,  the 
resulting  carbon  dioxide  and  water  must  be  deducted 
from  the  weights  found  before  making  the  final  cal- 
culations (see  p.  60). 


APPENDIX 

ATOMIC  WEIGHTS' 

Carbon     .         .         .         .         .         .         .         12.001 

Hydrogen    .  .         .         ,         .  .       1.0075 

Oxygen  (standard)  ....         16.000 

Nitrogen      .         .  .         .         .  .     14.045 

Chlorine  35-455 

Bromine       .  .    79.955 
Iodine       .......       126.85 

Sulphur        .......     32.065 

LOGARITHMIC  FACTORS2 

Carbon  in  carbon  dioxide          .  .  i  .43573 

Hydrogen  in  water  .  .  .       1.04883 

One  gram  of  sucrose,  Cl2H22Olt,  yields : 

0.5791  gram  H2O.  log.  factor  =  9.76272  —  10. 

1.5430  grams  CO2.          log.  factor  =  0.18836. 

One  gram  of  benzoic  acid,  C6H5COOH,  yields : 

0.4428  gram  H2O.  log.  factor  =  9.64622  —  10. 

2.5235  grams  CO2.          log.  factor  =  0.40201. 

One  gram  of  naphthalene,  CioH8,  yields : 

0.5627  gram  H2O.  log.  factor  =  9.75025—10. 

3-4357  grams  COa.          log.  factor  =  0.53602. 

1  Compiled  by  T.  W.  Richards  in  the  Proceedings  of  the  American  Academy 
of  Arts  and  Sciences,  April,  1899. 

2  Atomic  weights  given  above  are  here  used. 


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